machinst x64: have cmov modify its destination operand;

This is tricky: the control flow implicitly implied by the operand makes
it so that the output register may be undefined, if we mark it only as a
"def". Make it a "mod" instead, which matches our usage in the codebase,
and will make it crash if the output operand isn't unconditionally
defined before the instruction.

cranelift/codegen/src/isa/x64/inst/mod.rs

@@ -905,7 +905,7 @@ fn x64_get_regs(inst: &Inst, collector: &mut RegUsageCollector) {
         }
         Inst::Cmove { src, dst, .. } => {
             src.get_regs_as_uses(collector);
-            collector.add_def(*dst);
+            collector.add_mod(*dst);
         }
         Inst::Push64 { src } => {
             src.get_regs_as_uses(collector);
@@ -1126,7 +1126,7 @@ fn x64_map_regs<RUM: RegUsageMapper>(inst: &mut Inst, mapper: &RUM) {
             ..
         } => {
             src.map_uses(mapper);
-            map_def(mapper, dst)
+            map_mod(mapper, dst)
         }
         Inst::Push64 { ref mut src } => src.map_uses(mapper),
         Inst::Pop64 { ref mut dst } => {