[NFC][test] Adding fastcc test case for promoted 16-bit integer bitcasts.

The following: https://reviews.llvm.org/D82552

fixed an assert in the SelectionDag ISel legalizer for some CCs on armv7.

I noticed that this fix also fixes the assert when using fastcc, so I am
adding a fastcc regression test here.

Differential Revision: https://reviews.llvm.org/D82443