[GISel]: Enhance the MachineIRBuilder API

[GISel]: Enhance the MachineIRBuilder API

Allows the MachineIRBuilder APIs to directly create registers (based on
LLT or TargetRegisterClass) as well as accept MachineInstrBuilders
and implicitly converts to register(with getOperand(0).getReg()).

Eg usage:
LLT s32 = LLT::scalar(32);
auto C32 = Builder.buildConstant(s32, 32);
auto Tmp = Builder.buildInstr(TargetOpcode::G_SUB, s32, C32,
OtherReg);
auto Tmp2 = Builder.buildInstr(Opcode, DstReg,
Builder.buildConstant(s32, 31)); ....

Only a few methods added for now.

Reviewed by Tim

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@307302 91177308-0d34-0410-b5e6-96231b3b80d8