[AMDGPU] Simplify use of hasMovrel and hasVGPRIndexMode (#105680)

The generic subtarget has neither of these features. Rather than forcing
HasMovrel on, it is simpler to expand dynamic vector indexing to a
sequence of compare/select instructions.

NFC for real subtargets.

Author: jayfoad

llvm/lib/Target/AMDGPU/GCNSubtarget.cpp

@@ -143,14 +143,8 @@ GCNSubtarget &GCNSubtarget::initializeSubtargetDependencies(const Triple &TT,
   if (LDSBankCount == 0)
     LDSBankCount = 32;
 
-  if (TT.getArch() == Triple::amdgcn) {
-    if (LocalMemorySize == 0)
-      LocalMemorySize = 32768;
-
-    // Do something sensible for unspecified target.
-    if (!HasMovrel && !HasVGPRIndexMode)
-      HasMovrel = true;
-  }
+  if (TT.getArch() == Triple::amdgcn && LocalMemorySize == 0)
+    LocalMemorySize = 32768;
 
   AddressableLocalMemorySize = LocalMemorySize;
 
@@ -366,7 +360,7 @@ bool GCNSubtarget::hasMadF16() const {
 }
 
 bool GCNSubtarget::useVGPRIndexMode() const {
-  return !hasMovrel() || (EnableVGPRIndexMode && hasVGPRIndexMode());
+  return hasVGPRIndexMode() && (!hasMovrel() || EnableVGPRIndexMode);
 }
 
 bool GCNSubtarget::useAA() const { return UseAA; }

llvm/lib/Target/AMDGPU/SIISelLowering.cpp

@@ -13350,12 +13350,15 @@ bool SITargetLowering::shouldExpandVectorDynExt(unsigned EltSize,
 
   // On some architectures (GFX9) movrel is not available and it's better
   // to expand.
-  if (!Subtarget->hasMovrel())
+  if (Subtarget->useVGPRIndexMode())
     return NumInsts <= 16;
 
   // If movrel is available, use it instead of expanding for vector of 8
   // elements.
-  return NumInsts <= 15;
+  if (Subtarget->hasMovrel())
+    return NumInsts <= 15;
+
+  return true;
 }
 
 bool SITargetLowering::shouldExpandVectorDynExt(SDNode *N) const {