[SROA] Fix PR24463, a crash I introduced in SROA by allowing it to

handle more allocas with loads past the end of the alloca.

I suspect there are some related crashers with slightly different
patterns, but I'll fix those and add test cases as I find them.

Thanks to David Majnemer for the excellent test case reduction here.
Made this super simple to debug and fix.

llvm-svn: 246289