[RegUsageInfoCollector] Bugfix for handling of register aliases.

Don't assume the alias of a defined reg is always already in the set.

As the test case in https://bugs.llvm.org/show_bug.cgi?id=36587 discovered,
it is wrong to assume that all the aliases of the defined register in the
*current function* is already present in the UsedPhysRegsMask.

This patch changes this so that any definition in the current function of a
phys-reg always results in all its aliases inserted into the set of defined
registers.

Review: Quentin Colombet
https://reviews.llvm.org/D45157

llvm-svn: 331509