[RISCV] Remove special case for constant shift amount in FSHL/FSHR lowering to FSL/FSR.

Remove fshl/fshr with constant shift amount isel patterns. Replace
with fsr/fsl with constant isel patterns.

This hack was trying to preserve as much optimization opportunity
for fshl/fshr by constant as possible, but the conversion to
RISCVISD::FSR/FSL happens so late it probably isn't worth much.

The new isel patterns are needed by D117468 anyway.

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -2792,8 +2792,6 @@ 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 to prevent
     // accidentally setting the extra bit.

llvm/lib/Target/RISCV/RISCVInstrInfoZb.td

@@ -906,16 +906,11 @@ def : Pat<(riscv_fsl GPR:$rs1, GPR:$rs3, GPR:$rs2),
           (FSL GPR:$rs1, GPR:$rs2, GPR:$rs3)>;
 def : Pat<(riscv_fsr GPR:$rs1, GPR:$rs3, GPR:$rs2),
           (FSR GPR:$rs1, GPR:$rs2, GPR:$rs3)>;
-
-// fshl and fshr concatenate their operands in the same order. fsr and fsl
-// instruction use different orders. fshl will return its first operand for
-// shift of zero, fshr will return its second operand. fsl and fsr both return
-// $rs1 so the patterns need to have different operand orders.
-def : Pat<(fshr GPR:$rs3, GPR:$rs1, uimmlog2xlen:$shamt),
+def : Pat<(riscv_fsr GPR:$rs1, GPR:$rs3, uimmlog2xlen:$shamt),
           (FSRI GPR:$rs1, GPR:$rs3, uimmlog2xlen:$shamt)>;
-// We can use FSRI for fshl by immediate if we subtract the immediate from
+// We can use FSRI for FSL by immediate if we subtract the immediate from
 // XLen and swap the operands.
-def : Pat<(fshl GPR:$rs3, GPR:$rs1, uimmlog2xlen:$shamt),
+def : Pat<(riscv_fsl GPR:$rs3, GPR:$rs1, uimmlog2xlen:$shamt),
           (FSRI GPR:$rs1, GPR:$rs3, (ImmSubFromXLen uimmlog2xlen:$shamt))>;
 } // Predicates = [HasStdExtZbt]