[DebugInfo] MCP: collect and update DBG_VALUEs encountered in local block

MCP currently uses changeDebugValuesDefReg / collectDebugValues to find
debug users of a register, however those functions assume that all
DBG_VALUEs immediately follow the specified instruction, which isn't
necessarily true. This is going to become very often untrue when we turn
off CodeGenPrepare::placeDbgValues.

Instead of calling changeDebugValuesDefReg on an instruction to change its
debug users, in this patch we instead collect DBG_VALUEs of copies as we
iterate over insns, and update the debug users of copies that are made
dead. This isn't a non-functional change, because MCP will now update
DBG_VALUEs that aren't immediately after a copy, but refer to the same
register. I've hijacked the regression test for PR38773 to test for this
new behaviour, an entirely new test seemed overkill.

Differential Revision: https://reviews.llvm.org/D56265

llvm-svn: 368835