[AMDGPU] WQM: Allow insertion of exact mode transition as terminator

Allow WQM pass to insert transitions to exact mode among block
terminators, instead of forcing them to occur before terminators.

This should not yield any functional change, but allows block
splitting of control flow, such as that in D145329.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D151797