[SPIRV] Pointer legalization fails due to unimplemented downcasts

[winocm]

It looks like the pointer legalization bitcast pass fails horrifically if there is a GEP instruction used before that does not end up feeding into the actual load. Stores also similarly fail.

Here is a minimal test case reproducing this issue:

%struct.Uniforms = type { %struct.Matrix, %struct.Matrix }
%struct.Matrix = type { [4 x <4 x float>] }

define void @foo(ptr addrspace(2) %0) {
  %2 = getelementptr inbounds %struct.Uniforms, ptr addrspace(2) %0, i64 0, i32 0, i32 0, i64 10
  %3 = load volatile <4 x float>, ptr addrspace(2) %0, align 16
  ret void
}

define void @foo2(ptr addrspace(2) %0) {
  %2 = getelementptr inbounds %struct.Uniforms, ptr addrspace(2) %0, i64 0, i32 0, i32 0, i64 10
  store volatile <4 x float> splat (float 0.0), ptr addrspace(2) %0, align 16
  ret void
}

[winocm]

On the note of logical mode, the following example compiles, but is semantically wrong:

define void @foo(ptr addrspace(2) %0) {
  %2 = getelementptr i64, ptr addrspace(2) %0, i32 3
  %3 = load volatile i64, ptr addrspace(2) %2, align 8
  ret void
}

The GEP offset for the load gets dropped here.

llc on aarch64 gives me:

	.build_version macos, 15, 0
	.section	__TEXT,__text,regular,pure_instructions
	.globl	_foo                            ; -- Begin function foo
	.p2align	2
_foo:                                   ; @foo
	.cfi_startproc
; %bb.0:
	ldr	xzr, [x0, #24]
	ret
	.cfi_endproc

But llc with the spirv-- triple gives me:

	OpCapability Shader
	OpCapability Int64
	OpCapability Linkage
	OpMemoryModel Logical GLSL450
	OpSource Unknown 0
	OpName %6 "foo"
	OpDecorate %6 LinkageAttributes "foo" Export
	%1 = OpTypeInt 64 0
	%2 = OpTypePointer UniformConstant %1
	%3 = OpTypeVoid
	%4 = OpTypeFunction %3 %2
	%5 = OpTypeInt 32 0
	%6 = OpFunction %3 None %4              ; -- Begin function foo
	%7 = OpFunctionParameter %2
	%10 = OpLabel
	%8 = OpAccessChain %2 %7
	%9 = OpLoad %1 %8 Volatile|Aligned 8
	OpReturn
	OpFunctionEnd