[DirectX] DXILCBufferAccess pass is removing globals without replacing all uses

[Icohedron]

Discussed issue #141673 with @bogner and came to the conclusion that the underlying issue is that the DXILCBufferAccess pass is removing globals from the module without replacing all their uses, thus resulting in invalid IR.

This notably occurs when the memcpy intrinsic uses a global, as seen in #141673 and in this reproduction: https://godbolt.org/z/dn8fYd6bx
In this reproduction, the constant buffer global @someGlobal is being used by memcpy despite @someGlobal not being defined in the module.

To resolve this:

• DXILCBufferAccess should be made to handle memcpy intrinsics
• A check should be added to ensure that all uses of a global handle are replaced before the global handle is erased from the module (to ensure issues like this don't happen again silently)