[InstCombine] fix datalayout in test file

The only part of the datalayout that should matter for these tests
is the part that specifies the legal int widths ('n*'). But there
was a bug - that part of the string was not correctly separated with
the expected '-' character, so we were testing as if there were no
legal int widths at all. Removed the leading cruft so we have some 
legal ints to test with.

I noticed this while testing a potential change to the way we 
transform shifts and sexts in D42424.

llvm-svn: 323377