libclc: Rename [add|sub]_sat.ll to [add|sub]_sat_if.ll

configure.py allows overloading *.cl with *.ll, but will only ever build
the first file listed in SOURCES of ${file}.cl and ${file}.ll

add_sat, sub_sat, (and the soon to be submitted clz) all define interfaces in
${function_name}.ll which are implemented in ${function_name}_impl.ll.

Renaming the interface files is enough to get them to build again, fixing
CL usage of these functions.

Tested on clover/r600g.

Patch by: Aaron Watry

llvm-svn: 185000