[llvm] [RISCV] Move the rest of Zfa FLI instruction handling to lowerConstantFP. (PR #109217)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

This patch is not NFC.
```
; bin/llc -mtriple=riscv64 -mattr=+zfa,+d test.ll -o -
define float @test_float(float %x) nounwind {
entry:
  %cmp = fcmp ult float %x, 0.000000e+00
  %sel = select i1 %cmp, float -5.000000e-01, float 5.000000e-01
  ret float %sel
}

define double @test_double(double %x) nounwind {
entry:
  %cmp = fcmp ult double %x, 0.000000e+00
  %sel = select i1 %cmp, double -5.000000e-01, double 5.000000e-01
  ret double %sel
}
```
Before:
```
test_float:                             # @test_float
# %bb.0:                                # %entry
        fmv.w.x fa5, zero
        fle.s   a0, fa5, fa0
        fli.s   fa0, 0.5
        bnez    a0, .LBB0_2
# %bb.1:
        fneg.s  fa0, fa0
.LBB0_2:                                # %entry
        ret

test_double:                            # @test_double
# %bb.0:                                # %entry
        fmv.d.x fa5, zero
        fle.d   a0, fa5, fa0
        fli.d   fa0, 0.5
        bnez    a0, .LBB1_2
# %bb.1:
        fneg.d  fa0, fa0
.LBB1_2:                                # %entry
        ret
```
After:
```
test_float:                             # @test_float
# %bb.0:                                # %entry
        fmv.w.x fa5, zero
        fle.s   a0, fa5, fa0
        fli.s   fa0, 0.5
        bnez    a0, .LBB0_2
# %bb.1:
        fneg.s  fa0, fa0
.LBB0_2:                                # %entry
        ret

.LCPI1_0:
        .quad   0x3fe0000000000000              # double 0.5
        .quad   0xbfe0000000000000              # double -0.5
        .text
        .globl  test_double
        .p2align        2
        .type   test_double, at function
test_double:                            # @test_double
# %bb.0:                                # %entry
        fmv.d.x fa5, zero
        fle.d   a0, fa5, fa0
        xori    a0, a0, 1
        slli    a0, a0, 3
        lui     a1, %hi(.LCPI1_0)
        addi    a1, a1, %lo(.LCPI1_0)
        add     a0, a1, a0
        fld     fa0, 0(a0)
        ret
```


https://github.com/llvm/llvm-project/pull/109217