[AMDGPU] enable Hot Block Register Renaming by default for CI/CT run

This flips the default of -amdgpu-enable-hot-block-reg-renaming to true
to exercise the pass across large CI/CT builds. This is a temporary
enablement to flush out issues; users can still disable with
-mllvm -amdgpu-enable-hot-block-reg-renaming=false.

Author: michaelselehov

llvm/lib/Target/AMDGPU/AMDGPUTargetMachine.cpp

@@ -416,7 +416,7 @@ static cl::opt<bool> EnableRegReassign(
 static cl::opt<bool> EnableHotBlockRegRenaming(
     "amdgpu-enable-hot-block-reg-renaming",
     cl::desc("Enable hot block register renaming to reduce value density"),
-    cl::init(false), cl::Hidden);
+    cl::init(true), cl::Hidden);
 
 static cl::opt<bool> OptVGPRLiveRange(
     "amdgpu-opt-vgpr-liverange",