[DAGCombiner] Fix wrong folding of a build_vector into a blend with zero.

Method 'visitBUILD_VECTOR' in the DAGCombiner knows how to combine a
build_vector of a bunch of extract_vector_elt nodes and constant zero nodes
into a shuffle blend with a zero vector.

However, method 'visitBUILD_VECTOR' forgot that a floating point
build_vector may contain negative zero as well as positive zero.

Example:

define <2 x double> @example(<2 x double> %A) {
entry:
  %0 = extractelement <2 x double> %A, i32 0
  %1 = insertelement <2 x double> undef, double %0, i32 0
  %2 = insertelement <2 x double> %1, double -0.0, i32 1
  ret <2 x double> %2
}

Before this patch, llc (with -mattr=+sse4.1) wrongly generated
  movq   %xmm0, %xmm0  # xmm0 = xmm0[0],zero

So, the sign bit of the negative zero was effectively lost.

This patch fixes the problem by adding explicit checks for positive zero.

With this patch, llc produces the following code for the example above:
  movhpd .LCPI0_0(%rip), %xmm0

where .LCPI0_0 referes to a 'double -0'.

llvm-svn: 239070