Fix SIMD temp register interference (#58990)

Fix SIMD temp register interference (#58990)

On Windows/x86, if a call returns a SIMD8 in two registers
(eax, edx), and needs to be assembled and stored into a single
SIMD register, AND we don't have the SSE41 ABI available (because
the hardware doesn't support it, or we specifically disable it),
then we need a temporary register. In the failure case, the target
register and temporary register were the same, and then interfered
with each other. The fix is to force the temporary register to
be different from the target register.

Fixes #58899

Co-authored-by: Bruce Forstall <brucefo@microsoft.com>