Do not CopyFromReg physregs for live-in values. Instead, create a vreg for

Do not CopyFromReg physregs for live-in values.  Instead, create a vreg for
each live in, and copy the regs from the vregs.  As the very first thing we
do in the function, insert copies from the pregs to the vregs.  This fixes
problems where the token chain of CopyFromReg was not enough to allow reordering
of the copyfromreg nodes and other unchained nodes (e.g. div, which clobbers
eax on intel).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@21932 91177308-0d34-0410-b5e6-96231b3b80d8