[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