[InstCombine][X86] Expand MOVMSK to generic IR (PR39927)

First step towards removing the MOVMSK intrinsics completely - this patch expands MOVMSK to the pattern:

e.g. PMOVMSKB(v16i8 x):
%cmp = icmp slt <16 x i8> %x, zeroinitializer
%int = bitcast <16 x i8> %cmp to i16
%res = zext i16 %int to i32

Which is correctly handled by ISel and FastIsel (give or take an annoying movzx move....): https://godbolt.org/z/rkrSFW

Differential Revision: https://reviews.llvm.org/D60256

llvm-svn: 357909