[InstCombine] Fold shuffled intrinsic operands with constant operands (#141300)

We currently pull shuffles through binops and intrinsics, which is an
important canonical form for VectorCombine to be able to scalarize
vector sequences. But while binops can be folded with a constant
operand, intrinsics currently require all operands to be shufflevectors.

This extends intrinsic folding to be in line with regular binops by
reusing the constant "unshuffling" logic.

As far as I can tell the list of currently folded intrinsics don't
require any special UB handling.

This change in combination with #138095 and #137823 fixes the following
C:

```c
void max(int *x, int *y, int n) {
  for (int i = 0; i < n; i++)
    x[i] += *y > 42 ? *y : 42;
}
```

Not using the splatted vector form on RISC-V with `-O3 -march=rva23u64`:

```asm
	vmv.s.x	v8, a4
	li	a4, 42
	vmax.vx	v10, v8, a4
	vrgather.vi	v8, v10, 0
.LBB0_9:                                # %vector.body
                                        # =>This Inner Loop Header: Depth=1
	vl2re32.v	v10, (a5)
	vadd.vv	v10, v10, v8
	vs2r.v	v10, (a5)
```

i.e., it now generates

```asm
        li	a6, 42
        max	a6, a4, a6
.LBB0_9:                                # %vector.body
                                        # =>This Inner Loop Header: Depth=1
	vl2re32.v	v8, (a5)
	vadd.vx	v8, v8, a6
	vs2r.v	v8, (a5)
```

0 files changed, 0 insertions, 0 deletions