This patch allows `r7` to be used, regardless of its use as a frame pointer, as
a temporary register when popping `lr`, and also falls back to using a high
temporary register if, for some reason, we weren't able to find a suitable low
one.
Differential revision: https://reviews.llvm.org/D40961
Fixes https://bugs.llvm.org/show_bug.cgi?id=35481
llvm-svn: 321989
d17dabca31