[X86] Don't turn (c?-v:v) into (c?-v:0) by blindly using PSIGN.

Currently, we sometimes miscompile this vector pattern:
    (c ? -v : v)
We lower it to (because "c" is <4 x i1>, lowered as a vector mask):
    (~c & v) | (c & -v)

When we have SSSE3, we incorrectly lower that to PSIGN, which does:
    (c < 0 ? -v : c > 0 ? v : 0)
in other words, when c is either all-ones or all-zero:
    (c ? -v : 0)
While this is an old bug, it rarely triggers because the PSIGN combine
is too sensitive to operand order. This will be improved separately.

Note that the PSIGN tests are also incorrect. Consider:
    %b.lobit = ashr <4 x i32> %b, <i32 31, i32 31, i32 31, i32 31>
    %sub = sub nsw <4 x i32> zeroinitializer, %a
    %0 = xor <4 x i32> %b.lobit, <i32 -1, i32 -1, i32 -1, i32 -1>
    %1 = and <4 x i32> %a, %0
    %2 = and <4 x i32> %b.lobit, %sub
    %cond = or <4 x i32> %1, %2
    ret <4 x i32> %cond
if %b is zero:
    %b.lobit = <4 x i32> zeroinitializer
    %sub = sub nsw <4 x i32> zeroinitializer, %a
    %0 = <4 x i32> <i32 -1, i32 -1, i32 -1, i32 -1>
    %1 = <4 x i32> %a
    %2 = <4 x i32> zeroinitializer
    %cond = or <4 x i32> %a, zeroinitializer
    ret <4 x i32> %a
whereas we currently generate:
    psignd %xmm1, %xmm0
    retq
which returns 0, as %xmm1 is 0.

Instead, use a pure logic sequence, as described in:
https://graphics.stanford.edu/~seander/bithacks.html#ConditionalNegate

Fixes PR26110.

Differential Revision: http://reviews.llvm.org/D17181

llvm-svn: 261023