Fix #2609 - Handle out-of-bounds scalar replacements. (#2767)

* Fix #2609 - Handle out-of-bounds scalar replacements.

When SROA tries to do a replacement for an OpAccessChain that is exactly
one element out of bounds, the code was trying to access its internal
array of replacements and segfaulting.

This protects the code from doing this, and it additionally fixes the
way SROA works by not returning failure when it refuses to do a
replacement.  Instead of failing the optimization pass, SROA will now
simply refuse to do the replacement and keep going.

Additionally, this patch fixes the SROA logic to now return a proper status so we can
correctly state that the pass made no changes to the IR if it only found
invalid references.