[RISCV] Teach RISCVTargetLowering::isFPImmLegal about fli+fneg (#149075)

There was a mismatch between isFPImmlegal and the cases that are handled
by lowerConstantFP. isFPImmLegal didn't check for the case where we
support `fli` of a negated constant (and so can lower to fli+fneg). This
has very minimal impact (42 insertion, 47 deletions across an
rv22u64_zfa llvm-test-suite build including SPEC CPU 2017) but is added
here for completeness.

See the PR thread #149075 for furrther discussion about the degree to which isFPImmLegal and lowerConstantFP are consistent. We ultimately agreed it makes sense to add fli+fneg, but there may be other future cases where it doesn't make sense to match.

Author: asb

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -2319,6 +2319,10 @@ bool RISCVTargetLowering::isFPImmLegal(const APFloat &Imm, EVT VT,
   if (getLegalZfaFPImm(Imm, VT) >= 0)
     return true;
 
+  // Some constants can be produced by fli+fneg.
+  if (Imm.isNegative() && getLegalZfaFPImm(-Imm, VT) >= 0)
+    return true;
+
   // Cannot create a 64 bit floating-point immediate value for rv32.
   if (Subtarget.getXLen() < VT.getScalarSizeInBits()) {
     // td can handle +0.0 or -0.0 already.