[RegAlloc]  Avoid compile time regression with multiple copy hints.

As a fix for https://bugs.llvm.org/show_bug.cgi?id=40986 ("excessive
compile time building opencollada"), this patch makes sure that no
phys reg is hinted more than once from getRegAllocationHints().

This handles the case were many virtual registers are assigned to the
same physreg. The previous compile time fix (r343686) in
weightCalcHelper() only made sure that physical/virtual registers are
passed no more than once to addRegAllocationHint().

Review: Dimitry Andric, Quentin Colombet
https://reviews.llvm.org/D59201