[VPlan] Append recipes created via builder to worklist
The previous PR appended the top most created recipe to the worklist, and this PR extends it to any other nested recipes that were created, similar to InstCombine.
This removes the header mask in a good few more places on RISC-V as measured on SPEC CPU 2017, e.g. for the following loop:
```c
long f(const int *p, const int *q, long n) {
long a = 0, b = 0;
for (long i = 0;; i++) {
if (p[i] && q[i]) { a += i; b += i; }
if (i + 1 == n) break;
}
return a + b;
}
```
Before:
```asm
LBB0_1:
sh2add a4, a3, a0
vsetvli a5, a2, e32, m1, ta, ma
vle32.v v16, (a4)
vmsne.vi v0, v16, 0
sh2add a4, a3, a1
vsetvli zero, zero, e64, m2, ta, ma
vmsltu.vx v16, v10, a5
vle32.v v17, (a4), v0.t
vsetvli zero, zero, e32, m1, ta, ma
vmsne.vi v17, v17, 0
vmand.mm v17, v0, v17
vmand.mm v0, v16, v17
vsetvli zero, zero, e64, m2, tu, mu
vadd.vv v8, v14, v8, v0.t
vadd.vv v12, v14, v12, v0.t
vsetvli zero, zero, e64, m2, ta, ma
vadd.vx v14, v14, a5
sub a2, a2, a5
add a3, a3, a5
bnez a2, .LBB0_1
```
After:
```
.LBB0_1:
sh2add a4, a3, a0
vsetvli a5, a2, e32, m1, ta, ma
vle32.v v14, (a4)
vmsne.vi v0, v14, 0
sh2add a4, a3, a1
vle32.v v14, (a4), v0.t
vmsne.vi v14, v14, 0
vmand.mm v0, v0, v14
vsetvli zero, zero, e64, m2, tu, mu
vadd.vv v8, v12, v8, v0.t
vadd.vv v10, v12, v10, v0.t
vsetvli zero, zero, e64, m2, ta, ma
vadd.vx v12, v12, a5
sub a2, a2, a5
add a3, a3, a5
bnez a2, .LBB0_1
```
This copies the IRBuilderInstCombineInserter for IRBuilder approach from InstCombine, but templating VPBuilder + adding a VPBuilderBase seemed overkill so I've just made a dervied class with a virtual method instead, which does the same thing.
We can also remove the (X && Y) | (X && !Y) -> X combine, but this is left for another NFC PR.
