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

@@ -1367,9 +1367,15 @@
 
 (decl cmp_and_choose (Type CC Value Value) ValueRegs)
 (rule (cmp_and_choose (fits_in_64 ty) cc x y)
-      (let ((size OperandSize (raw_operand_size_of_type ty)))
-        (with_flags_reg (x64_cmp size x y)
-                        (cmove ty cc y x))))
+      (let ((size OperandSize (raw_operand_size_of_type ty))
+            ;; We need to put x and y in registers explicitly because
+            ;; we use the values more than once. Hence, even if these
+            ;; are "unique uses" at the CLIF level and would otherwise
+            ;; allow for load-op merging, here we cannot do that.
+            (x_reg Reg x)
+            (y_reg Reg y))
+        (with_flags_reg (x64_cmp size x_reg y_reg)
+                        (cmove ty cc y_reg x_reg))))
 
 (rule (lower (has_type (fits_in_64 ty) (umin x y)))
       (cmp_and_choose ty (CC.B) x y))

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