[AMDGPU] Change getLdStRegisterOperand to !cond for better diagnostic (#95475)

If you would hit the unexpected case in these !if trees, you'd get an
error message like "error: Not a known RegisterClass! def VReg_1..."
This can happen when changing code quite indirectly related to these
class definitions. We can use !cond here, which has a builtin facility
to throw an error if no case in the !cond statement is hit.

NFC.

Author: Sisyph

llvm/lib/Target/AMDGPU/BUFInstructions.td

@@ -399,12 +399,10 @@ class MUBUF_Invalidate <string opName, SDPatternOperator node = null_frag> :
 
 class getLdStVDataRegisterOperand<RegisterClass RC, bit isTFE> {
   RegisterOperand tfeVDataOp =
-    !if(!eq(RC.Size, 32), AVLdSt_64,
-    !if(!eq(RC.Size, 64), AVLdSt_96,
-    !if(!eq(RC.Size, 96), AVLdSt_128,
-    !if(!eq(RC.Size, 128), AVLdSt_160,
-    RegisterOperand<VReg_1>  // Invalid register.
-    ))));
+    !cond(!eq(RC.Size, 32)  : AVLdSt_64,
+          !eq(RC.Size, 64)  : AVLdSt_96,
+          !eq(RC.Size, 96)  : AVLdSt_128,
+          !eq(RC.Size, 128) : AVLdSt_160);
 
   RegisterOperand ret = !if(isTFE, tfeVDataOp, getLdStRegisterOperand<RC>.ret);
 }

llvm/lib/Target/AMDGPU/SIInstrInfo.td

@@ -2242,13 +2242,11 @@ class getHasExt <int NumSrcArgs, ValueType DstVT = i32, ValueType Src0VT = i32,
 // Return an AGPR+VGPR operand class for the given VGPR register class.
 class getLdStRegisterOperand<RegisterClass RC> {
   RegisterOperand ret =
-    !if(!eq(RC.Size, 32), AVLdSt_32,
-      !if(!eq(RC.Size, 64), AVLdSt_64,
-        !if(!eq(RC.Size, 96), AVLdSt_96,
-          !if(!eq(RC.Size, 128), AVLdSt_128,
-            !if(!eq(RC.Size, 160), AVLdSt_160,
-              RegisterOperand<VReg_1> // invalid register
-    )))));
+    !cond(!eq(RC.Size, 32)   : AVLdSt_32,
+          !eq(RC.Size, 64)   : AVLdSt_64,
+          !eq(RC.Size, 96)   : AVLdSt_96,
+          !eq(RC.Size, 128)  : AVLdSt_128,
+          !eq(RC.Size, 160)  : AVLdSt_160);
 }
 
 class getHasVOP3DPP <ValueType DstVT = i32, ValueType Src0VT = i32,