[InstCombine] reduce funnel-shift i16 X, X, 8 to bswap X

Prefer the more exact intrinsic to remove a use of the input value
and possibly make further transforms easier (we will still need
to match patterns with funnel-shift of wider types as pieces of
bswap, especially if we want to canonicalize to funnel-shift with
constant shift amount). Discussed in D46760.

llvm-svn: 364187