This makes front/back symmetric with begin/end, avoiding some confusion.
Added instr_front/instr_back for the old behavior, corresponding to
instr_begin/instr_end. Audited all three in-tree users of back(), all
of them look like they don't want to look inside bundles.
Fixes an assertion (PR19815) when generating debug info on mips, where a
delay slot was bundled at the end of a branch.
llvm-svn: 209580