[DAGCombine] Don't fold a trunc if it feeds an anyext

Legalization tends to create anyext(trunc) patterns. This should always be
combined - into either a single trunc, a single ext, or nothing if the
types match exactly. But if we happen to combine the trunc first, we may pull
the trunc away from the anyext or make it implicit (e.g. the truncate(extract)
-> extract(bitcast) fold).

To prevent this, we can avoid doing the fold, similarly to how we already handle
fpround(fpextend).

Differential Revision: https://reviews.llvm.org/D23893

llvm-svn: 280386