IR: Don't add inbounds to GEPs of extern_weak variables
Global variables that have `extern_weak` linkage may be null, so it's
incorrect to add `inbounds` when constant folding.
This also fixes a bug when parsing global aliases, whose forward
reference placeholders are global variables with `extern_weak` linkage.
If GEPs to these aliases are encountered before the alias itself, the
GEPs would incorrectly gain the `inbounds` keyword as well.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215803
91177308-0d34-0410-b5e6-
96231b3b80d8