[GlobalISel] Allow prelegalizer combiners to have access to LegalizerInfo.

Before, the isPreLegalize() query in CombinerHelper only checked for the
presence of a LegalizerInfo object. This is problematic when we want to have
a combine actually check for legality in a pre-legalizer combine pass, since
if we pass a LegalizerInfo object to the constructor it causes the combines to
think that we're running *post* legalizer, which isn't true.

This change fixes it to instead check an explicit bool that passes to signal
whether the pass will be run before or after legalization.

Doing so exposed a bug in the extending loads combine, which tried to check for
legality of candidate extending loads if LegalizerInfo was present. Since we
only ran it pre-legalizer and therefore with a null LegalizerInfo, it never
actually ran. Also fixes the legality checks to keep the tests passing.

Differential Revision: https://reviews.llvm.org/D135044