Add a PrintRegUnit helper similar to PrintReg.

Reg-units are named after their root registers, and most units have a
single root, so they simply print as 'AL', 'XMM0', etc. The rare dual
root reg-units print as FPSCR~FPSCR_NZCV, FP0~ST7, ...

The printing piggybacks on the existing register name tables, so no
extra const data space is required.

llvm-svn: 157754