This PR seeks to expand/replace the Constant -> Instruction conversion
that needs to occur inside of the OpenMP Target kernel generation to
allow kernel argument replacement of uses within the kernel (cannot
replace constant uses within constant expressions with non-constants).
It does so by making use of the new-ish utility
convertUsersOfConstantsToInstructions which is a much more expansive
version of what the smaller "version" of the function I wrote does,
effectively expanding uses of the input argument that are constant
expressions into instructions so that we can replace with the
appropriate kernel argument.
Also alters convertUsersOfConstantsToInstructions to optionally
restrict the replacement to a function and optionally leave
dead constants alone, the latter is necessary when lowering from
MLIR as we cannot be sure we can remove the constants at this
stage, even if rewritten to instructions the ModuleTranslation may
maintain links to the original constants and utilise them in further
lowering steps (as when we're lowering the kernel, the module is
still in the process of being lowered). This can result in unusual
ICEs later. These dead constants can be tidied up later (and
appear to be in subsequent lowering from checking with
emit-llvm).
0aeaa2d93d