[AMDGPU] Improve killed check for vgpr optimization

The killed flag is not always set. E.g. when a variable is used in a
loop, it is never marked as killed, although it is unused in following
basic blocks. Also, we try to deprecate kill flags and not use them.

Check if the register is live in the endif block. If not, consider it
killed in the then and else blocks.

The vgpr-liverange tests have two new tests with loops
(pre-committed, so the diff is visible).
I also needed to change the subtarget to gfx10.1, otherwise calls
are not working.

Differential Revision: https://reviews.llvm.org/D106291