[TargetLowering][DAGCombine][MSP430] add/use hook for Shift Amount Threshold (1/2)

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 PR43559:
https://bugs.llvm.org/show_bug.cgi?id=43559

Patch by: @joanlluch (Joan LLuch)

Differential Revision: https://reviews.llvm.org/D69116

llvm-svn: 375347