This is a continuation of D68982, following community recommendations
Provides a TLI hook to allow targets to relax the emission of shifts, thus enabling codegen improvements on targets with no multiple shift instructions and cheap selects or branches.
Contributes to a Fix for https://bugs.llvm.org/show_bug.cgi?id=43559
Should be commited after D69099
This is part 1, defining the TLI hook, incorporating it in the MSP430 target, and using it in TagetLogwering.cpp.