[RISCV] Handle recurrences in VLOptimizer

[lukel97]

With EVL tail folding given a loop with an induction variable:

void f(int *x, int n) {
  for (int i = 0; i < n; i++)
    x[i] += i;
}

We will have an extra vsetvli toggle because we can't currently reduce the VL of the induction increment, which is used by a phi node forming a cycle:

.LBB0_2:                                # %vector.body
                                        # =>This Inner Loop Header: Depth=1
	sub	a3, a1, a2
	sh2add	a4, a2, a0
	vsetvli	a3, a3, e32, m2, ta, ma
	vle32.v	v10, (a4)
	add	a2, a2, a3
	vadd.vv	v10, v10, v8
	vse32.v	v10, (a4)
	vsetvli	a4, zero, e32, m2, ta, ma
	vadd.vx	v8, v8, a3
	bne	a2, a1, .LBB0_2

I have a patch that I plan on posting shortly that handles this, based off the work here: https://www.youtube.com/watch?v=Mfb5fRSdJAc

[llvmbot]

@llvm/issue-subscribers-backend-risc-v

Author: Luke Lau (lukel97)

With EVL tail folding given a loop with an induction variable:

void f(int *x, int n) {
  for (int i = 0; i < n; i++)
    x[i] += i;
}

We will have an extra vsetvli toggle because we can't currently reduce the VL of the induction increment, which is used by a phi node forming a cycle:

.LBB0_2:                                # %vector.body
                                        # =>This Inner Loop Header: Depth=1
	sub	a3, a1, a2
	sh2add	a4, a2, a0
	vsetvli	a3, a3, e32, m2, ta, ma
	vle32.v	v10, (a4)
	add	a2, a2, a3
	vadd.vv	v10, v10, v8
	vse32.v	v10, (a4)
	vsetvli	a4, zero, e32, m2, ta, ma
	vadd.vx	v8, v8, a3
	bne	a2, a1, .LBB0_2

I have a patch that I plan on posting shortly that handles this, based off the work here: https://www.youtube.com/watch?v=Mfb5fRSdJAc