[X86] Covert unsigned int 0 to float-point with FILD instruction.

unsinged int 0 will be convert to float/double -0.0 when the rounding
mode is set to 'FE_DOWNWARD'. Using FILD instruction instead of SSE
instructions on 32-bit target if the strictfp is enabled.

Differential Revision: https://reviews.llvm.org/D123660