[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