[X86][SSE] Improve SINT_TO_FP of boolean vector results (signum)

This patch helps avoids poor legalization of boolean vector results (e.g. 8f32 -> 8i1 -> 8i16) that feed into SINT_TO_FP by inserting an early SIGN_EXTEND and so help improve the truncation logic.

This is not necessary for AVX512 targets where boolean vectors are legal - AVX512 manages to lower ( sint_to_fp vXi1 ) into some form of ( select mask, 1.0f , 0.0f ) in most cases.

Fix for PR13248

Differential Revision: https://reviews.llvm.org/D26583

llvm-svn: 286979