[X86] Legalize zero extends from vXi1 to vXi16/vXi32/vXi64 using a sign extend and a shift.

This avoids a constant pool load to create 1.

The int->float are showing converts to mask and back. We probably need to widen inputs to sint_to_fp/uint_to_fp before type legalization.

llvm-svn: 324805