[SROA] prevent crash on large memset length (PR50910)

I don't know much about this pass, but we need a stronger
check on the memset length arg to avoid an assert. The
current code was added with D59000.
The test is reduced from:
https://llvm.org/PR50910

Differential Revision: https://reviews.llvm.org/D106462

(cherry picked from commit f2a322bf)