Simplify a bunch of isVirtualRegister() and isPhysicalRegister() logic.

These functions not longer assert when passed 0, but simply return false instead.

No functional change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@123155 91177308-0d34-0410-b5e6-96231b3b80d8
20 files changed