[AMDGPU][MC] Fixed bugs in export instruction

See Bugs 33019, 33056:
  https://bugs.llvm.org//show_bug.cgi?id=33019
  https://bugs.llvm.org//show_bug.cgi?id=33056

Reviewers: artem.tamazov, vpykhtin

Differential Revision: https://reviews.llvm.org/D33288

llvm-svn: 303423