Make clz/ctz builtins defined for zero on ARM targets. rdar://10732455

Make clz/ctz builtins defined for zero on ARM targets.  rdar://10732455

ARM supports clz and ctz directly and both operations have well-defined
results for zero.  There is no disadvantage in performance to using the
defined-at-zero versions of llvm.ctlz/cttz intrinsics.  We're running into
ARM-specific code written with the assumption that __builtin_clz(0) == 32,
even though that value is technically undefined.  The code is failing now
because of llvm optimizations that are taking advantage of the undef
behavior (specifically svn r147255).  There's nothing wrong with that
optimization on x86 where any incorrect assumptions about __builtin_clz(0)
will quickly be exposed.  For ARM, though, optimizations based on that undef
behavior are likely to cause subtle bugs.  Other targets with defined-at-zero
clz/ctz support may want to override the default behavior as well.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@149086 91177308-0d34-0410-b5e6-96231b3b80d8