[LV] Fix for PR38110, LV encountered llvm_unreachable()

Summary: truncateToMinimalBitWidths() doesn't handle all Instructions and the worst case is compiler crash via llvm_unreachable(). Fix is to add a case to handle PHINode and changed the worst case to NO-OP (from compiler crash).

Reviewers: sbaranga, mssimpso, hsaito

Reviewed By: hsaito

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D49461

llvm-svn: 337861