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 f2a322bfcfbc62b5523f32c4eded6faf2cad2e24)