It doesn't seem useful and in several locations we already use a
different Opcode from what the user actually wrote. For example for
tls_value, the implicit stackprobe or the implicit memcpy call for
struct args.

This also makes the recorded call sites more accurately represent the
actual set of call sites by avoiding special casing of pseudo
instructions that happen to include a call.

Instead remove the actual set of defs from the clobber set once they are
known. This reduces the risk of getting the clobber set wrong and makes
it easier to handle cases where the exact set is not known in advance.