[Hexagon] Give uniform names to functions changing addressing modes, NFC

The new format is changeAddrMode_xx_yy, where xx is the current mode,
and yy is the new one.

Old name:               New name:
getBaseWithImmOffset    changeAddrMode_abs_io
getAbsoluteForm         changeAddrMode_io_abs
getBaseWithRegOffset    changeAddrMode_io_rr
xformRegToImmOffset     changeAddrMode_rr_io
getBaseWithLongOffset   changeAddrMode_rr_ur
getRegShlForm           changeAddrMode_ur_rr

llvm-svn: 315013