[X86] Remove MVT::i1 handling code from LowerTRUNCATE

Summary: I don't think this is necessary with i1 being illegal now.

Reviewers: RKSimon, zvi, guyblank

Reviewed By: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38784

llvm-svn: 315469