Change the arg lowering code to use copyfromreg from vregs associated

Change the arg lowering code to use copyfromreg from vregs associated
with incoming arguments instead of the pregs themselves.  This fixes
the scheduler from causing problems by moving a copyfromreg for an argument
to after a select_cc node (now it can, and bad things won't happen).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@23334 91177308-0d34-0410-b5e6-96231b3b80d8