Don't invoke getName() from Function::isIntrinsic().

Summary:
getName() involves a hashtable lookup, so is expensive given how
frequently isIntrinsic() is called.  (In particular, many users cast to
IntrinsicInstr or one of its subclasses before calling
getIntrinsicID().)

This has an incidental functional change: Before, isIntrinsic() would
return true for any function whose name started with "llvm.", even if it
wasn't properly an intrinsic.  The new behavior seems more correct to
me, because it's strange to say that isIntrinsic() is true, but
getIntrinsicId() returns "not an intrinsic".

Some callers want the old behavior -- they want to know whether the
caller is a recognized intrinsic, or might be one in some other version
of LLVM.  For them, we added Function::hasLLVMReservedName(), which
checks whether the name starts with "llvm.".

This change is good for a 1.5% e2e speedup compiling a large Eigen
benchmark.

Reviewers: bogner

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D22065

llvm-svn: 276942