[CodeGenPrepare] Increase the limit on the number of instructions to scan

... when finding all memory uses for an address and make it a
parameter.

Now that we have avoided potentially exponential run time of
`FindAllMemoryUses` in D143893. it'd be beneficial to increase the
limit up from 20.

Reviewed By: mkazantsev

Differential Revision: https://reviews.llvm.org/D143894

Change-Id: I3abdf40332ef65e9b2f819ac32ac60e4200ec51d