[DirectX] Support ConstExpr GEPs

[kmpeng]

The issue still needs to be filed, but this PR partially addresses the Data Scalarization pass failing to handle nested constantexpr GEPs.

Farzon wants to rethink this solution to make it recursive, so once the issue is filed/that solution is put up, this PR will be closed.

[Icohedron]

Note: The pointer operand of a ConstantExpr GEP can be another ConstantExpr GEP.

Example:

llvm-project/llvm/test/CodeGen/DirectX/flatten-array.ll

Lines 136 to 142 in 66e707e

	define void @global_nested_geps() {
	; CHECK-LABEL: define void @global_nested_geps(
	; CHECK: {{.*}} = load i32, ptr getelementptr inbounds ([24 x i32], ptr @a.1dim, i32 0, i32 6), align 4
	; CHECK-NEXT: ret void
	%1 = load i32, i32* getelementptr inbounds ([4 x i32], [4 x i32]* getelementptr inbounds ([3 x [4 x i32]], [3 x [4 x i32]]* getelementptr inbounds ([2 x [3 x [4 x i32]]], [2 x [3 x [4 x i32]]]* @a, i32 0, i32 0), i32 0, i32 1), i32 0, i32 2), align 4
	ret void
	}

[farzonl]

I was afraid of that. Then we will have to make this recursive for constexprs. Which sucks that we will have to do this twice once for data scalarization and then again for flattening. I'm wondering if it wouldn't just make more sense to undo all the constExprs before these passes?

[Icohedron]

I don't think it will ever naturally occur though. It may suffice to assert that the ConstantExpr GEP's pointer operand is a global variable and deal with it later if it does become an actual problem.

AFAIK there isn't a good reason to codegen a multiply-nested ConstantExpr GEP in the first place when you can codegen a single ConstantExpr GEP instead because all the indices are ConstantInts.

[farzonl]

This PR supercedes this one and will fix the multilple nesting case: #150082

[farzonl]

This change does fix a bug, but I think we need to file a different ticket for this case:

groupshared uint4 aTile[10][10];

[numthreads(1, 1, 1)]
void CSMain() {
    uint4 aFragPacked = aTile[1][2];
}

Need to rename the branch and then we don't have to call this a partial fix.