[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

llvm-svn: 355854