Simplify SROA conversion to integer in some ways, make it more general in others.

We now tolerate small amounts of undefined behavior, better emulating what
would happen if the transaction actually occurred in memory.  This fixes
SingleSource/UnitTests/2007-04-10-BitfieldTest.c on PPC, at least until
Devang gets a chance to fix the CFE from doing undefined things with bitfields :)

llvm-svn: 35875