[GlobalIsel][X86] support G_TRUNC selection.

Summary:
[GlobalIsel][X86] support G_TRUNC selection.
Add regbank-select and legalizer tests. Currently legalization of trunc i64 on 32bit platform not supported.

Reviewers: ab, zvi, rovka

Reviewed By: zvi

Subscribers: dberris, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D32115

llvm-svn: 300678