[NFC][SPIRV] Small changes for SPIRVPushConstantAccess (#174734)
This patch consists of 3 NFC changes:
* Do not always report _changed_, report it only when something was
modified
* Replace loop with `replaceUsesOfWith`
* Instead of using a temporary vector, use `make_early_inc_range()` to
pre-increment the iterator before erasing the current element.