[InstCombine] add (sext i1 X), 1 --> zext (not X)

http://rise4fun.com/Alive/i8Q

A narrow bitwise logic op is obviously better than math for value tracking, 
and zext is better than sext. Typically, the 'not' will be folded into an 
icmp predicate.

The IR difference would even survive through codegen for x86, so we would see 
worse code:

https://godbolt.org/g/C14HMF

one_or_zero(int, int):                      # @one_or_zero(int, int)
        xorl    %eax, %eax
        cmpl    %esi, %edi
        setle   %al
        retq

one_or_zero_alt(int, int):                  # @one_or_zero_alt(int, int)
        xorl    %ecx, %ecx
        cmpl    %esi, %edi
        setg    %cl
        movl    $1, %eax
        subl    %ecx, %eax
        retq

llvm-svn: 306243