intel/compiler: Support fmul_fsign opt for fp64 when int64 isn't supported

intel/compiler: Support fmul_fsign opt for fp64 when int64 isn't supported

MTL support fp64, but not int64. The fsign(double(x))*FOO optimization
would try to use a 64-bit int xor operation to conditionally toggle
the sign bit off the result.

Since this only affects high bit of the result, we can do a 32-bit
move of the low dword, and a 32-bit xor on the high dword.

Fixes dEQP-VK.spirv_assembly.instruction.compute.float_controls.fp64.input_args.modf_denorm_flush_to_zero
on MTL.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22259>