findPHICopyInsertPoint special cases placement in a block with a
callbr or invoke in it. In that case, we must ensure that the copy is
placed before the INLINEASM_BR or call instruction, if the register is
defined prior to that instruction, because it may jump out of the
Previously, the code placed it immediately after the last def _or
use_. This is wrong, if the use is the instruction which may jump. We
could correctly place it immediately after the last def (ignoring
uses), but that is non-optimal for register pressure.
Instead, place the copy after the last def, or before the
call/inlineasm_br, whichever is later.