[NFC] Make getPreferredAlignment honor section markings.

This should more accurately reflect what the AsmPrinter will actually
do.

This is NFC, as far as I can tell; all the places that might be affected
already have an extra check to avoid using the result of
getPreferredAlignment in this situation.

Differential Revision: https://reviews.llvm.org/D51377

llvm-svn: 340999