[AMDGPU] Fix data race in SIInsertWaitcnts

The race condition happened when two pass managers ran on two different modules but modified/read the global variables.

To address this, I considered using singletons and freestanding functions to allow getting/setting `HardwareLimits` and `RegisterEncoding`, or making it local to the pass. I chose the latter and made it a member of `WaitcntsBrackets`, to minimizes the amount of global state.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D115896