[GlobalISel][X86] extend G_ZEXT support.

Summary:
Mark G_ZEXT/G_SEXT i1 to i8/i16,  i8 to i16 as legal.
Support G_ZEXT i1 to i8/i16 instruction selection ( C++ code).
This patch requred to support G_LOAD/G_STORE i1.

Reviewers: zvi, guyblank

Reviewed By: guyblank

Subscribers: rovka, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D35177

llvm-svn: 307526