[X86] Correct the width of mask arguments in intrinsic headers and tests.

All of these found by grepping through IR from the builtin tests for extra trunc and zext/sext instructions that shouldn't have been there.

Some of these were real bugs where we lost bits from the user input:
_mm512_mask_broadcast_f32x8
_mm512_maskz_broadcast_f32x8
_mm512_mask_broadcast_i32x8
_mm512_maskz_broadcast_i32x8
_mm256_mask_cvtusepi16_storeu_epi8

llvm-svn: 336042