[AMDGPU] Add hot block register renaming pass

This patch introduces a post-allocation register renaming optimization
pass that reduces value density in hot basic blocks. The pass helps
the post-RA scheduler avoid false WAW dependencies by moving local
values to unused physical registers.

The pass operates after greedy register allocation but before
VirtRegRewriter. It identifies hot blocks (above frequency threshold),
calculates value density per physical register, and selectively moves
local live ranges to free registers. Only 32-bit VGPR values that live
entirely within a single basic block are moved, ensuring conservative
behavior.

Key features:
- Respects tied operands and register allocation constraints
- Honors occupancy-based VGPR limits to avoid spilling
- Disabled by default (enable with -amdgpu-enable-hot-block-reg-renaming)
- Includes comprehensive lit tests

Performance results show up to 2% improvement on register-intensive
kernels such as rocRAND MTGP32.

Author: michaelselehov

llvm/lib/Target/AMDGPU/AMDGPU.h

@@ -545,6 +545,9 @@ extern char &AMDGPUExportKernelRuntimeHandlesLegacyID;
 void initializeGCNNSAReassignLegacyPass(PassRegistry &);
 extern char &GCNNSAReassignID;
 
+void initializeAMDGPUHotBlockRegisterRenamingLegacyPass(PassRegistry &);
+extern char &AMDGPUHotBlockRegisterRenamingID;
+
 void initializeGCNPreRALongBranchRegLegacyPass(PassRegistry &);
 extern char &GCNPreRALongBranchRegID;