[llvm] [AMDGPU] Added hot-block-rematerialize pass (PR #126331)

[Adam Yang via llvm-commits]

adam-yang wrote:

> Here's one for `-amdgpu-remat-enable-hot-block-remat-aggressive` which fails an assertion "Invalid RC for virtual register": [r2.txt](https://github.com/user-attachments/files/19468032/r2.txt)

This revealed some incorrect assumptions in this PR. Lanes are assumed to be 32-bit, but new 16-bit subregs have been added so lanes are now 16-bit. I'll have to go through the change to fix some more issues related to this.

https://github.com/llvm/llvm-project/pull/126331