[DAGCombiner] make sure we have a whole-number extract before trying to narrow...

[DAGCombiner] make sure we have a whole-number extract before trying to narrow a vector op (PR39511)

The test causes a crash because we were trying to extract v4f32 to v3f32, and the
narrowing factor was then 4/3 = 1 producing a bogus narrow type.

This should fix:
https://bugs.llvm.org/show_bug.cgi?id=39511

llvm-svn: 345842