gallivm: Always use floating-point operators for floating-point types

gallivm: Always use floating-point operators for floating-point types

This fixes the assert added in LLVM 2.8:
assert(getType()->isIntOrIntVectorTy() &&
       "Tried to create an integer operation on a non-integer type!")

But it also fixes some subtle bugs, since we should've been doing this
since LLVM 2.6 anyway.

Includes a modified patch from steckdenis@yahoo.fr for the
FNeg instructions in emit_fetch(); thanks for pointing those out.

http://bugs.freedesktop.org/29404
http://bugs.freedesktop.org/29407

Signed-off-by: José Fonseca <jfonseca@vmware.com>