[RISCV] Add helper method for detecting BEXTI or TH_TST is supported. NFC (#157915)

These instructions both extract single bit to bit 0 and fill the upper
bits with 0.

There's at least one place where we check for BEXTI but not TH_TST. I
wanted to keep this patch NFC so that will be a follow up fix.

Author: topperc

llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp

@@ -1052,7 +1052,7 @@ void RISCVDAGToDAGISel::Select(SDNode *Node) {
   SDLoc DL(Node);
   MVT VT = Node->getSimpleValueType(0);
 
-  bool HasBitTest = Subtarget->hasStdExtZbs() || Subtarget->hasVendorXTHeadBs();
+  bool HasBitTest = Subtarget->hasBEXTILike();
 
   switch (Opcode) {
   case ISD::Constant: {

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -2173,7 +2173,7 @@ bool RISCVTargetLowering::isMaskAndCmp0FoldingBeneficial(
   // on the basis that it's possible the sinking+duplication of the AND in
   // CodeGenPrepare triggered by this hook wouldn't decrease the instruction
   // count and would increase code size (e.g. ANDI+BNEZ => BEXTI+BNEZ).
-  if (!Subtarget.hasStdExtZbs() && !Subtarget.hasVendorXTHeadBs())
+  if (!Subtarget.hasBEXTILike())
     return false;
   ConstantInt *Mask = dyn_cast<ConstantInt>(AndI.getOperand(1));
   if (!Mask)

llvm/lib/Target/RISCV/RISCVSubtarget.h

@@ -186,6 +186,8 @@ class RISCVSubtarget : public RISCVGenSubtargetInfo {
     return HasStdExtZfhmin || HasStdExtZfbfmin;
   }
 
+  bool hasBEXTILike() const { return HasStdExtZbs || HasVendorXTHeadBs; }
+
   bool hasCZEROLike() const {
     return HasStdExtZicond || HasVendorXVentanaCondOps;
   }