[X86, AVX] adjust tablegen patterns to generate better code for scalar...

[X86, AVX] adjust tablegen patterns to generate better code for scalar insertion into zero vector (PR23073)

For code like this:

define <8 x i32> @load_v8i32() {
  ret <8 x i32> <i32 7, i32 0, i32 0, i32 0, i32 0, i32 0, i32 0, i32 0>
}

We produce this AVX code:

_load_v8i32:                            ## @load_v8i32
  movl	$7, %eax
  vmovd	%eax, %xmm0
  vxorps	%ymm1, %ymm1, %ymm1
  vblendps	$1, %ymm0, %ymm1, %ymm0 ## ymm0 = ymm0[0],ymm1[1,2,3,4,5,6,7]
  retq

There are at least 2 bugs in play here:

    We're generating a blend when a move scalar does the same job using 2 less instruction bytes (see FIXMEs).
    We're not matching an existing pattern that would eliminate the xor and blend entirely. The zero bytes are free with vmovd.

The 2nd fix involves an adjustment of "AddedComplexity" [1] and mostly masks the 1st problem.

[1] AddedComplexity has close to no documentation in the source. 
The best we have is this comment: "roughly corresponds to the number of nodes that are covered". 
It appears that x86 has bastardized this definition by inflating its values for some other
undocumented reason. For example, we have a pattern with "AddedComplexity = 400" (!). 

I searched my way to this page:
https://groups.google.com/forum/#!topic/llvm-dev/5UX-Og9M0xQ

Differential Revision: http://reviews.llvm.org/D8794

llvm-svn: 233931