[RISCV][ISelLowering] Use Zicond for FP selects on Zfinx/Zdinx (#169299)
### Summary
This patch let RISCVTargetLowering::lowerSELECT to lower some
floating-point select operations through an integer zicond select when:
* Zicond is available, and
* FP values live in GPRs (Zfinx/Zdinx), and
* Select condition is an integer type.
In that scenario there is no extra cost for GPR <-> "FP GPR" moves, so
we can implement FP selects with a CZERO-based sequence instead of a
branch.
For example, for
```c
float foo(int cond, float x) {
return (cond != 0) ? x : 0.0f;
}
```
the current lowering produces:
```asm
foo:
mv a2, a0
li a0, 0
beqz a2, .LBB0_2
.LBB0_1:
mv a0, a1
.LBB0_2:
ret
```
With this patch, when targeting rv64ima_zicond_zfinx we instead get:
```asm
foo:
czero.nez a2, zero, a0
czero.eqz a0, a1, a0
or a0, a2, a0
ret
```
The existing branch-based lowering is preserved for:
* targets without Zicond
* targets where FP registers are separate (+f, +d without zfinx/zdinx)
### Testing
Adds llvm/test/CodeGen/RISCV/zicond-fp-select-zfinx.ll to cover:
* RV64 Zfinx/Zicond vs Zfinx without Zicond
* RV64 Zdinx/Zicond vs Zdinx without Zicond
* RV32 Zfinx/Zicond vs Zfinx without Zicond
Also adds baseline RV32F/RV64F/RV64D cases to ensure we still use
branches when FP registers are separate.
The tests check that:
* With Zicond + Zfinx/Zdinx, FP select lowers to a CZERO+OR sequence
with no conditional branches.
* Without Zicond (or without Zfinx/Zdinx), we still get branch-based
code and no czero.* instructions.
llvm-project
2e21bb81 - [RISCV][ISelLowering] Use Zicond for FP selects on Zfinx/Zdinx (#169299)
