[VPlan] Optimize LastActiveLane to EVL - 1 (#169766)
With EVL tail folding, the LastActiveLane can be computed with EVL - 1.
This removes the need for a header mask and vfirst.m for loops with live
outs on RISC-V:
# %bb.5: # %for.cond.cleanup7
- vsetvli zero, zero, e32, m2, ta, ma
- vmv.v.x v8, s1
- vmsleu.vv v10, v8, v22
- vfirst.m a0, v10
- srli a1, a0, 63
- czero.nez a0, a0, a1
- czero.eqz a1, s8, a1
- or a0, a0, a1
- addi a0, a0, -1
- vsetvli zero, zero, e64, m4, ta, ma
- vslidedown.vx v8, v12, a0
+ addi s1, s1, -1
+ vslidedown.vx v8, v12, s1
1c7ec06b16