Make dense maps keyed on physical registers smallerusing

Make dense maps keyed on physical registers smallerusing
MRegisterInfo::getNumRegs() instead of
MRegisterInfo::FirstVirtualRegister.

Also use MRegisterInfo::is{Physical,Virtual}Register where
appropriate.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@11477 91177308-0d34-0410-b5e6-96231b3b80d8