GlobalISel: give MachineInstrBuilder a uniform interface. NFC.

Instead of an ad-hoc collection of "buildInstr" functions with varying numbers
of registers, this uses variadic templates to provide for as many regs as
needed!

Also make IRtranslator use new "buildBr" function instead of some weird generic
one that no-one else would really use.

llvm-svn: 276762