X86: Fix LowerBUILD_VECTORAsVariablePermute for case Src is smaller than Indices
As RKSimon suggested in pr35820, in the case that Src is smaller in
bit-size than Indices, need to widen Src to avoid type mismatch.
Reviewers: RKSimon, craig.topper
Reviewed By: RKSimon
Differential Revision: https://reviews.llvm.org/D41865