[AArch64][SVE] Improve fixed-length addressing modes. (#129732)
When compiling VLS SVE, the compiler often replaces VL-based offsets
with immediate-based ones. This leads to a mismatch in the allowed
addressing modes due to SVE loads/stores generally expecting immediate
offsets relative to VL. For example, given:
```c
svfloat64_t foo(const double *x) {
svbool_t pg = svptrue_b64();
return svld1_f64(pg, x+svcntd());
}
```
When compiled with `-msve-vector-bits=128`, we currently generate:
```gas
foo:
ptrue p0.d
mov x8, #2
ld1d { z0.d }, p0/z, [x0, x8, lsl #3]
ret
```
Instead, we could be generating:
```gas
foo:
ldr z0, [x0, #1, mul vl]
ret
```
Likewise for other types, stores, and other VLS lengths.
This patch achieves the above by extending `SelectAddrModeIndexedSVE`
to let constants through when `vscale` is known.
llvm-project
f01e760c - [AArch64][SVE] Improve fixed-length addressing modes. (#129732)
