[SelectionDAG] Assume that a GlobalAlias may alias other global values
This fixes a bug detected in DAGCombiner when using global alias
variables. Here is an example:
@foo = global i16 0, align 1
@aliasFoo = alias i16, i16 * @foo
define i16 @bar() {
...
store i16 7, i16 * @foo, align 1
store i16 8, i16 * @aliasFoo, align 1
...
}
BaseIndexOffset::computeAliasing would incorrectly derive NoAlias
for the two accesses in the example above, resulting in DAGCombiner
miscompiles.
This patch fixes the problem by a defensive approach letting
BaseIndexOffset::computeAliasing return false, i.e. that the aliasing
couldn't be determined, when comparing two global values and at least
one is a GlobalAlias. In the future we might improve this with a
deeper analysis to look at the aliasee for the GlobalAlias etc. But
that is a bit more complicated considering that we could have
'local_unnamed_addr' and situations with several 'alias' variables.
Fixes PR51878.
Differential Revision: https://reviews.llvm.org/D110064