[RISCV] Don't convert fshr/fshl to target specific FSL/FSR node if shift amount is a constant.

As long as it's a constant we can directly pattern match it
without any problems. It's only when it isn't a constant that
we need to add an AND.

In theory this should allow more target independent optimizations
to remain active.

Author: topperc

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -1558,6 +1558,8 @@ SDValue RISCVTargetLowering::LowerOperation(SDValue Op,
     MVT VT = Op.getSimpleValueType();
     assert(VT == Subtarget.getXLenVT() && "Unexpected custom legalization");
     SDLoc DL(Op);
+    if (Op.getOperand(2).getOpcode() == ISD::Constant)
+      return Op;
     // FSL/FSR take a log2(XLen)+1 bit shift amount but XLenVT FSHL/FSHR only
     // use log(XLen) bits. Mask the shift amount accordingly.
     unsigned ShAmtWidth = Subtarget.getXLen() - 1;

llvm/lib/Target/RISCV/RISCVInstrInfoB.td

@@ -758,11 +758,11 @@ def : Pat<(riscv_fsl GPR:$rs1, GPR:$rs3, GPR:$rs2),
 def : Pat<(riscv_fsr GPR:$rs3, GPR:$rs1, GPR:$rs2),
           (FSR GPR:$rs1, GPR:$rs2, GPR:$rs3)>;
 
-def : Pat<(riscv_fsr GPR:$rs3, GPR:$rs1, uimmlog2xlen:$shamt),
+def : Pat<(fshr GPR:$rs3, GPR:$rs1, uimmlog2xlen:$shamt),
           (FSRI GPR:$rs1, GPR:$rs3, uimmlog2xlen:$shamt)>;
 // We can use FSRI for fshl by immediate if we subtract the immediate from
 // XLen and swap the operands.
-def : Pat<(riscv_fsl GPR:$rs3, GPR:$rs1, uimmlog2xlen:$shamt),
+def : Pat<(fshl GPR:$rs3, GPR:$rs1, uimmlog2xlen:$shamt),
           (FSRI GPR:$rs1, GPR:$rs3, (ImmSubFromXLen uimmlog2xlen:$shamt))>;
 } // Predicates = [HasStdExtZbt]