stage1: Let LLVM legalize the overflowing ops on vectors

LLVM is smart enough to produce tight overflow checking sequences.

On x86_64 a simple v4u32 addition is turned from:

```
  c5 e9 d4 d4                  	vpaddq	%xmm4, %xmm2, %xmm2
  c5 f1 d4 cb                  	vpaddq	%xmm3, %xmm1, %xmm1
  c5 f8 28 e9                  	vmovaps	%xmm1, %xmm5
  c4 e3 55 18 ea 01            	vinsertf128	$1, %xmm2, %ymm5, %ymm5
  c5 f0 c6 da 88               	vshufps	$136, %xmm2, %xmm1, %xmm3
  c5 c8 57 f6                  	vxorps	%xmm6, %xmm6, %xmm6
  c4 e3 55 0c ee aa            	vblendps	$170, %ymm6, %ymm5, %ymm5
  c4 e3 7d 19 ec 01            	vextractf128	$1, %ymm5, %xmm4
  c4 e2 69 29 d4               	vpcmpeqq	%xmm4, %xmm2, %xmm2
  c5 e9 ef d0                  	vpxor	%xmm0, %xmm2, %xmm2
  c4 e2 71 29 cd               	vpcmpeqq	%xmm5, %xmm1, %xmm1
  c5 f1 ef c0                  	vpxor	%xmm0, %xmm1, %xmm0
  c5 f9 6b c2                  	vpackssdw	%xmm2, %xmm0, %xmm0
  c5 f8 50 c0                  	vmovmskps	%xmm0, %eax
  84 c0                        	testb	%al, %al
```

into:

```
  c5 f9 fe c9                  	vpaddd	%xmm1, %xmm0, %xmm1
  c4 e2 71 3f c0               	vpmaxud	%xmm0, %xmm1, %xmm0
  c5 f1 76 c0                  	vpcmpeqd	%xmm0, %xmm1, %xmm0
  c5 f8 50 c0                  	vmovmskps	%xmm0, %eax
  34 0f                        	xorb	$15, %al
  0f 95 c1                     	setne	%cl
  f6 c1 01                     	testb	$1, %cl
```

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