Teach computeRegisterProperties() to compute "representative" register class...

Teach computeRegisterProperties() to compute "representative" register class for legal value types. A "representative" register class is the largest legal super-reg register class for a value type. e.g. On i386, GR32 is the rep register class for i8 / i16 / i32; on x86_64 it would be GR64.

This property will be used by the register pressure tracking instruction scheduler.

llvm-svn: 108735