InstCombine: (icmp eq B, 0) | (icmp ult A, B) -> (icmp ule A, B-1)

This transform allows us to turn IR that looks like:
  %1 = icmp eq i64 %b, 0
  %2 = icmp ult i64 %a, %b
  %3 = or i1 %1, %2
  ret i1 %3

into:
  %0 = add i64 %b, -1
  %1 = icmp uge i64 %0, %a
  ret i1 %1

which means we go from lowering:
        cmpq    %rsi, %rdi
        setb    %cl
        testq   %rsi, %rsi
        sete    %al
        orb     %cl, %al
        ret

to lowering:
        decq    %rsi
        cmpq    %rdi, %rsi
        setae   %al
        ret

llvm-svn: 185677