X86: Fix LowerBUILD_VECTORAsVariablePermute for case Src is smaller than Indices

Summary:
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.

Fixes pr35820

Reviewers: RKSimon, craig.topper

Reviewed By: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D41865

llvm-svn: 322272