[CIR] Add get_element operation for computing pointer to array element

[tommymcm]

Overview

Currently, getting the pointer to an element of an array requires a pointer decay and a (possible) pointer stride. A similar pattern for records has been eliminated with the cir.get_member operation. This PR provides a similar level of abstraction for arrays with the get_element operation.
get_element replaces the above pattern with a single operation, which takes a pointer to an array and an index, and produces a pointer to the element at that index.
There are many places in CIR analysis and lowering where the ptr_stride(array_to_ptrdecay(x), i) pattern is handled as a special case. By subsuming the special case pattern with an explicit operation, we make these analyses and lowering more robust.

Changes

Adds the cir.get_element operation.
Extends CIRGen to emit cir.get_element for array subscript expressions.
Updated LifetimeCheck to handle get_element operation, subsuming special case analysis of cir.ptr_stride operation (did not remove the special case).
Extends CIR-to-LLVM lowering to lower cir.get_element to llvm.getelementptr
Extends CIR-to-MLIR lowering to lower cir.get_element to memref operations, matching existing special case cir.ptr_stride lowering.

Additional Notes

Currently, 47.6% of cir.ptr_stride operations in the llvm-test-suite (SingleSource and MultiSource) can be replaced by cir.get_element operations.

Operator Breakdown (current)

name	count	%
cir.load	825221	22.27%
cir.br	429822	11.60%
cir.const	380381	10.26%
cir.cast	325646	8.79%
cir.store	309586	8.35%
cir.get_member	226895	6.12%
cir.get_global	186851	5.04%
cir.ptr_stride	158094	4.27%
cir.call	144522	3.90%
cir.binop	141142	3.81%
cir.alloca	134346	3.63%
cir.brcond	112864	3.05%
cir.cmp	83532	2.25%

Operator Breakdown (with get_element)

name	count	%
cir.load	825221	22.74%
cir.br	429822	11.84%
cir.const	380381	10.48%
cir.store	309586	8.53%
cir.cast	248645	6.85%
cir.get_member	226895	6.25%
cir.get_global	186851	5.15%
cir.call	144522	3.98%
cir.binop	141142	3.89%
cir.alloca	134346	3.70%
cir.brcond	112864	3.11%
cir.cmp	83532	2.30%
cir.ptr_stride	81093	2.23%
cir.get_elem	77001	2.12%

[erichkeane]

I don't really have a comment and didn't come up with anything on my review, but IMO, this seems like a good idea.

[andykaylor]

I think this is a very good addition to the dialect.

[tommymcm]

Thanks for the review @andykaylor! I've applied the requested changes and responded to your comments above.

[andykaylor]

lgtm, with one last nit

[tommymcm]

Wicked, thanks for the help @andykaylor

[tommymcm]

I have rebased the PR after the incubator rebase this morning. Looks like there are some errors, digging into that now.

[tommymcm]

Rebased PR again, works with the new main branch.