[AMDGPU][MC] Added arg checks for vmcnt, expcnt, lgkmcnt helpers

Summary of changes:
- corrected vmcnt, expcnt, lgkmcnt helpers to checks their argument for truncation;
- added saturated versions of these helpers.

See bug 32711 for details: https://bugs.llvm.org//show_bug.cgi?id=32711

Reviewers: artem.tamazov, vpykhtin

Differential Revision: https://reviews.llvm.org/D32546

llvm-svn: 301439