x64 backend: fix fpcmp to avoid load-op merging. (#3934)

The `fpcmp` helper in the x64 backend uses `put_in_xmm_mem` for one of
its operands, which allows the compiler to merge a load with the compare
instruction (`ucomiss` or `ucomisd`).

Unfortunately, as we saw in #2576 for the integer-compare case, this
does not work with our lowering algorithm because compares can be
lowered more than once (unlike all other instructions) to reproduce the
flags where needed. Merging a load into an op that executes more than
once is invalid in general (the two loads may observe different values,
which violates the original program semantics because there was only one
load originally).

This does not result in a miscompilation, but instead will cause a panic
at regalloc time because the register that should have been defined by
the separate load is never written (the load is never emitted
separately).

I think this (very subtle, easy to miss) condition was unfortunately not
ported over when we moved the logic in #3682.

The existing fcmp-of-load test in `cmp-mem-bug` (from #2576) does not
seem to trigger it, for a reason I haven't fully deduced. I just added
the verbatim function body (happens to come from `clang.wasm`) that
triggers the bug as a test.

Discovered while bringing up regalloc2 support. It's pretty unlikely to
hit by chance, which is why I think none of our fuzzing has hit it yet.

cranelift/codegen/src/isa/x64/inst.isle

@@ -1528,9 +1528,11 @@
 ;; `clif.isle`).
 (decl fpcmp (Value Value) ProducesFlags)
 (rule (fpcmp src1 @ (value_type $F32) src2)
-      (xmm_cmp_rm_r (SseOpcode.Ucomiss) (put_in_xmm_mem src1) (put_in_xmm src2)))
+      ;; N.B.: cmp can be generated more than once, so cannot do a
+      ;; load-op merge. So `put_in_xmm` for src1, not `put_in_xmm_mem`.
+      (xmm_cmp_rm_r (SseOpcode.Ucomiss) (put_in_xmm src1) (put_in_xmm src2)))
 (rule (fpcmp src1 @ (value_type $F64) src2)
-      (xmm_cmp_rm_r (SseOpcode.Ucomisd) (put_in_xmm_mem src1) (put_in_xmm src2)))
+      (xmm_cmp_rm_r (SseOpcode.Ucomisd) (put_in_xmm src1) (put_in_xmm src2)))
 
 ;; Helper for creating `test` instructions.
 (decl test (OperandSize GprMemImm Gpr) ProducesFlags)

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

@@ -1,4 +1,4 @@
 src/clif.isle 9ea75a6f790b5c03
 src/prelude.isle b2bc986bcbbbb77
-src/isa/x64/inst.isle 40f495d3ca5ae547
+src/isa/x64/inst.isle cdd292107fb36cf
 src/isa/x64/lower.isle c049f7d36db0e0fb