[SLP] Peek into loads when hitting the RecursionMaxDepth

This patch slightly extends the limit on the RecursionMaxDepth inside
the SLP vectorizer. It does it only when it hits a load (or zext/sext of
a load), which allows it to peek through in the places where it will be
the most valuable, without ballooning out the O(..) by any 2^n factors.

Differential Revision: https://reviews.llvm.org/D122148