[AMDGPU] Use verify<domtree> instead of intra-pass asserts.

Verifying dominator tree is expensive using intra-pass
asserts. Asserts added during D147408 are
increasing the build time of libc significantly. This change
does the verification after the atomic optimizer pass
and should fix the regression reported in D153232.

Reviewed By: arsenm, #amdgpu

Differential Revision: https://reviews.llvm.org/D153261