stage2 x86_64: implement integer mul

This was also an experiment to see if it were easier to implement a new
feature when using the instruction encoder.

Verdict: It's not that much easier, but I think it's certainly much more
readable, because the description of the Instruction annotates what each
field means. Right now, precise knowledge of x86_64 instructions is
still required because things like when to set the 64-bit flag, how to
read x86_64 instruction references, etc. are still not automatically
done for you.

In the future, this interface might make it sligtly easier to write an
assembler for x86_64, by abstracting the bit-fiddling aspects of
instruction encoding.

src/Module.zig

@@ -4330,6 +4330,33 @@ pub fn intSub(allocator: *Allocator, lhs: Value, rhs: Value) !Value {
     }
 }
 
+pub fn intMul(allocator: *Allocator, lhs: Value, rhs: Value) !Value {
+    // TODO is this a performance issue? maybe we should try the operation without
+    // resorting to BigInt first.
+    var lhs_space: Value.BigIntSpace = undefined;
+    var rhs_space: Value.BigIntSpace = undefined;
+    const lhs_bigint = lhs.toBigInt(&lhs_space);
+    const rhs_bigint = rhs.toBigInt(&rhs_space);
+    const limbs = try allocator.alloc(
+        std.math.big.Limb,
+        lhs_bigint.limbs.len + rhs_bigint.limbs.len + 1,
+    );
+    var result_bigint = BigIntMutable{ .limbs = limbs, .positive = undefined, .len = undefined };
+    var limbs_buffer = try allocator.alloc(
+        std.math.big.Limb,
+        std.math.big.int.calcMulLimbsBufferLen(lhs_bigint.limbs.len, rhs_bigint.limbs.len, 1),
+    );
+    defer allocator.free(limbs_buffer);
+    result_bigint.mul(lhs_bigint, rhs_bigint, limbs_buffer, allocator);
+    const result_limbs = result_bigint.limbs[0..result_bigint.len];
+
+    if (result_bigint.positive) {
+        return Value.Tag.int_big_positive.create(allocator, result_limbs);
+    } else {
+        return Value.Tag.int_big_negative.create(allocator, result_limbs);
+    }
+}
+
 pub fn floatAdd(
     arena: *Allocator,
     float_type: Type,
@@ -4396,6 +4423,39 @@ pub fn floatSub(
     }
 }
 
+pub fn floatMul(
+    arena: *Allocator,
+    float_type: Type,
+    src: LazySrcLoc,
+    lhs: Value,
+    rhs: Value,
+) !Value {
+    switch (float_type.tag()) {
+        .f16 => {
+            @panic("TODO add __trunctfhf2 to compiler-rt");
+            //const lhs_val = lhs.toFloat(f16);
+            //const rhs_val = rhs.toFloat(f16);
+            //return Value.Tag.float_16.create(arena, lhs_val * rhs_val);
+        },
+        .f32 => {
+            const lhs_val = lhs.toFloat(f32);
+            const rhs_val = rhs.toFloat(f32);
+            return Value.Tag.float_32.create(arena, lhs_val * rhs_val);
+        },
+        .f64 => {
+            const lhs_val = lhs.toFloat(f64);
+            const rhs_val = rhs.toFloat(f64);
+            return Value.Tag.float_64.create(arena, lhs_val * rhs_val);
+        },
+        .f128, .comptime_float, .c_longdouble => {
+            const lhs_val = lhs.toFloat(f128);
+            const rhs_val = rhs.toFloat(f128);
+            return Value.Tag.float_128.create(arena, lhs_val * rhs_val);
+        },
+        else => unreachable,
+    }
+}
+
 pub fn simplePtrType(
     mod: *Module,
     arena: *Allocator,