Fix incorrect target triple in fp16-promote.ll

Summary:
Hyphens were missing from the triple, causing it to be parsed
incorrectly.  This patch updates the triple and makes necessary
changes to the expected output.

Patch is from Vinicius Tinti.

Reviewers: ab, tinti

Subscribers: srhines, llvm-commits

Differential Revision: http://reviews.llvm.org/D13792

llvm-svn: 251020