[GlobalISel][Legalizer] More concise and faster widenScalar, NFC

Refactoring LegalizerHelper::widenScalar member function reducing its
size by approximately a factor of 2 and (hopefuly) making it more
straightforward and regular by introducing widenScalarSrc and
widenScalarDst helper methods.

The new widenScalar* methods mutate the instructions in place instead
of recreating them from scratch and removing the originals. The
compile time implications of this were measured on sqlite3
amalgamation, targeting AArch64 in -O0:

LegalizerHelper::widenScalar: > 25% faster
Legalizer::runOnMachineFunction: ~ 4.0 - 4.5% faster

Also adding MachineOperand::setCImm and refactoring out
MachineIRBuilder::recordInsertion methods to make the change possible.

Reviewers: aditya_nandakumar, bogner, javed.absar, t.p.northover, ab, dsanders, arsenm

Reviewed By: aditya_nandakumar

Subscribers: wdng, rovka, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D46414

llvm-svn: 331819