[DAGCombiner] guard against an oversized shift crash

This change prevents the crash noted in the post-commit comments 
for rL347478 :
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20181119/605166.html

We can't guarantee that an oversized shift amount is folded away, 
so we have to check for it.

Note that I committed an incomplete fix for that crash with:
rL347502

But as discussed here:
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20181126/605679.html
...we have to try harder.

So I'm not sure how to expose the bug now (and apparently no fuzzers have found 
a way yet either).

On the plus side, we have discovered that we're missing real optimizations by 
not simplifying nodes sooner, so the earlier fix still has value, and there's 
likely more value in extending that so we can simplify more opcodes and simplify 
when doing RAUW and/or putting nodes on the combiner worklist.

Differential Revision: https://reviews.llvm.org/D54954

llvm-svn: 348089