[IR] Allow type change in ValueAsMetadata::handleRAUW (#76969)

`ValueAsMetadata::handleRAUW` is a mechanism to replace all metadata
referring to one value by a different value.

Relax an assert that used to enforce the old and new value to have the
same type.
This seems to be a sanity plausibility assert only, as the
implementation actually supports mismatching types.

This is motivated by a downstream mechanism where we use poison
ValueAsMetadata values to annotate pointee types of opaque pointer
function arguments.

When replacing one type with a different one to work around DXIL vs LLVM
incompatibilities, we need to update type annotations, and handleRAUW is
more efficient than creating new MD nodes.