[PowerPC] Add asm parser support for CR expressions

This adds support for specifying condition registers and
condition register fields via expressions using the symbols
defined by the PowerISA, like "4*cr2+eq".



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185633 91177308-0d34-0410-b5e6-96231b3b80d8