[TargetLowering][DAGCombine][MSP430] Shift Amount Threshold in DAGCombine (4)

Summary:
Replaces
```
unsigned getShiftAmountThreshold(EVT VT)
```
by

```
bool shouldAvoidTransformToShift(EVT VT, unsigned amount)
```
thus giving more flexibility for targets to decide whether particular shift amounts must be considered expensive or not.

Updates the MSP430 target with a custom implementation.

This continues  D69116, D69120, D69326 and updates them, so all of them must be committed before this.

Existing tests apply, a few more have been added.

Reviewers: asl, spatel

Reviewed By: spatel

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70042