[InstCombine] Combine interleaved recurrences. (#143878)
Combine sequences such as:
```llvm
%pn1 = phi [init1, %BB1], [%op1, %BB2]
%pn2 = phi [init2, %BB1], [%op2, %BB2]
%op1 = binop %pn1, constant1
%op2 = binop %pn2, constant2
%rdx = binop %op1, %op2
```
Into:
```llvm
%phi_combined = phi [init_combined, %BB1], [%op_combined, %BB2]
%rdx_combined = binop %phi_combined, constant_combined
```
This allows us to simplify interleaved reductions, for example as
introduced by the loop vectorizer.
The anecdotal example for this is the loop below:
```c
float foo() {
float q = 1.f;
for (int i = 0; i < 1000; ++i)
q *= .99f;
return q;
}
```
Which currently gets lowered explicitly such as (on AArch64,
interleaved by four):
```gas
.LBB0_1:
fmul v0.4s, v0.4s, v1.4s
fmul v2.4s, v2.4s, v1.4s
fmul v3.4s, v3.4s, v1.4s
fmul v4.4s, v4.4s, v1.4s
subs w8, w8, #32
b.ne .LBB0_1
```
But with this patch lowers trivially:
```gas
foo:
mov w8, #5028
movk w8, #14389, lsl #16
fmov s0, w8
ret
```
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