[llvm] Conditionally include target intrinsics, based on --enable-target

Some work to shrink the size of llvm binaries. Saves ~100KB. Avoid including Intrinsics-X.td files and generating code for targets that are not specified by --enable-target=X,Y,Z.

BEFORE: "size llc" w/ only x86
   text	   data	    bss	    dec	    hex	filename
11328502	 502632	  36976	11868110	 b517ce	Release+Asserts/bin/llc

AFTER:
   text	   data	    bss	    dec	    hex	filename
11232165	 499080	  36976	11768221	 b3919d	Release+Asserts/bin/llc



(a) This is done by adding "include_if" directive to TableGen (vs trying to do a full fledge #if def / #else / #endif).
(b) CPP defines for inline code the depends on these intrinsics (e.g., in InstCombine). See http://codereview.chromium.org/7824006/ for clang side changes. I have no idea about llvm-gcc or dragon egg dependencies on LLVM.
(c) move tests that are target dependent into target-specific directories. Previously tests would assume that "opt" understood target specific intrinsics.

TEST= build w/ --enable-targets=all, --enable-targets=x86, --enable-targets=arm, run "make check" in each case.

I had a bit of trouble w/ "make check" for --enable-target=arm in the case of JIT tests and generic tests, since I was running on an x86 machine.