x64 backend: fix a load-op merging bug with integer min/max. (#4068)

The recent work in #4061 introduced a notion of "unique uses" for CLIF
values that both simplified the load-op merging rules and allowed
loads to merge in some more places.

Unfortunately there's one factor that PR didn't account for: a unique
use at the CLIF level could become a multiple-use at the VCode level,
when a lowering uses a value multiple times!

Making this less error-prone in general is hard, because we don't know
the lowering in VCode until it's emitted, so we can't ahead-of-time
know that a value will be used multiple times and prevent its
merging. But we *can* know in the lowerings themselves when we're
doing this. At least we get a panic from regalloc when we get this
wrong; no bad code (uninitialized register being read) should ever
come from a backend bug like this.

This is still a bit less than ideal, but for now the fix is: in
`cmp_and_choose` in the x64 backend (which compares values, then
picks one or the other with a cmove), explicitly put values in
registers.

Fixes #4067 (thanks @Mrmaxmeier for the report!).

cranelift/codegen/src/isa/x64/lower/isle/generated_code.manifest

@@ -1,4 +1,4 @@
 src/clif.isle 443b34b797fc8ace
 src/prelude.isle afd037c4d91c875c
 src/isa/x64/inst.isle cad03431447aca1b
-src/isa/x64/lower.isle 803aac716f6f4c39
+src/isa/x64/lower.isle a7181571835ddd1e