Follow the implementation approach suggested by PR6687,

which generates more efficient and more obviously conformant
code.  We now test for overflow of the multiply then force
the result to -1 if so.  On X86, this generates nice code
like this:

__Z4testl:                              ## @_Z4testl
## BB#0:                                ## %entry
	subl	$12, %esp
	movl	$4, %eax
	mull	16(%esp)
	testl	%edx, %edx
	movl	$-1, %ecx
	cmovel	%eax, %ecx
	movl	%ecx, (%esp)
	call	__Znam
	addl	$12, %esp
	ret

llvm-svn: 108927