[X86] Prevent crash in LowerBUILD_VECTORvXi1 for v64i1 vectors on 32-bit...

[X86] Prevent crash in LowerBUILD_VECTORvXi1 for v64i1 vectors on 32-bit targets when the vector is a mix of constants and non-constant.

We need to materialize the constants as two 32-bit values that
are casted to v32i1 and then concatenated.

llvm-svn: 372304