[DAGCombiner] add operation legality checks before creating shift ops (PR43542)

As discussed on llvm-dev and:
https://bugs.llvm.org/show_bug.cgi?id=43542
...we have transforms that assume shift operations are legal and transforms to
use them are profitable, but that may not hold for simple targets.

In this case, the MSP430 target custom lowers shifts by repeating (many)
simpler/fixed ops. That can be avoided by keeping this code as setcc/select.

Differential Revision: https://reviews.llvm.org/D68397

llvm-svn: 373666