[Local] Simplify the alignment limits in getOrEnforceKnownAlignment. NFCI

We previously clamped the trailing zero count to 31 bits. And
then clamped the final alignment to MaximumAlignment which is
1 << 29.

This patch simplifies this to just clamp the trailing zero to
29 using MaxAlignmentExponent.

I was looking into changing this function to use Align/MaybeAlign
and noticed this.

Differential Revision: https://reviews.llvm.org/D78418