make simplifycfg insert an llvm.trap before the 'unreachable' it introduces

when it detects undefined behavior.  llvm.trap generally codegens into some
thing really small (e.g. a 2 byte ud2 instruction on x86) and debugging this
sort of thing is "nontrivial".  For example, we now compile:

void foo() { *(int*)0 = 42; }

into:

_foo:
	pushl	%ebp
	movl	%esp, %ebp
	ud2

Some may even claim that this is a security hole, though that seems dubious
to me.  This addresses rdar://7958343 - Optimizing away null dereference 
potentially allows arbitrary code execution


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@103356 91177308-0d34-0410-b5e6-96231b3b80d8

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