AMDGPU: Fix breaking IR on instructions with multiple pointer operands

The promote alloca pass would attempt to promote an alloca with
a select, icmp, or phi user, even though the other operand was
from a non-promotable source, producing a select on two different
pointer types.

Only do this if we know that both operands derive from the same
alloca. In the future we should be able to relax this to an alloca
which will also be promoted.

llvm-svn: 269265