[X86] Lower zext i1 arguments

i1 is now a legal type for X86 with AVX512.
There were some paths in X86FastISel which were not quite ready to see
an i1 value: they were not quite sure how to deal with sign/zero extends
for call arguments.
DTRT by extending to i8 for zeroext and bailing out of FastISel for
signext.

This fixes PR27591.

llvm-svn: 268470