[LSROA] Array support

We should update the LSROA pass to analyze loaded indices of arrays and only extract those.

The current LSROA implementation only splits load/stores on structs, not arrays. This means if a global array is copied in a temporary and a single element is loaded, the full array may be copied (for example if it's in a different address space, like it is in SPIR-V).

Acceptance Criteria

Given some IR with a structured.alloc of an array, where it loads and stores to individual elements via structured.gep:

define i32 @test_simple_array() {
entry:
  %tmp = call elementtype([10 x i32]) ptr @llvm.structured.alloca.p0()
  %ptr = call ptr (ptr, ...) @llvm.structured.gep.p0(ptr elementtype([10 x i32]) %tmp, i32 0)
  store i32 0, ptr %ptr
  %res = load i32, ptr %ptr
  ret i32 %res
}

We should be able to elminate the structured.gep and operate on the individual elements directly:

define i32 @test_simple_array() {
entry:
  %tmp_i32 = call elementtype(i32) ptr @llvm.structured.alloca.p0()
  store i32 0, ptr %tmp_i32
  %res = load i32, ptr %tmp_i32
  ret i32 %res
}

We will need to update the existing LSROA/array.ll test case to show that this works.