I have an issue where a SLP tree hit the RecursionMaxDepth limit in the SLP vectorizer, and is unable to analyze the whole tree because of it. This patch effectively slightly extends the limit, but only when it hits a load (or zext/sext of a load). This 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.
The compile time I measured for this (and D122145 together, as percentage differences) were small:
ClamAV 0.086 7zip -0.012 tramp3d-v4 0.098 kimwitu++ 0.008 sqlite3 0.046 mafft 0.003 lencod 0.169 SPASS 0.006 consumer-typeset -0.019 Bullet 0.011
typo nit: should be "peek"