[Clang] VectorExprEvaluator::VisitCallExpr / InterpretBuiltin - allow SSE41 phminposuw intrinsic to be used in constexp

[RKSimon]

_mm_minpos_epu16

index[2:0] := 0
min[15:0] := a[15:0]
FOR j := 0 to 7
	i := j*16
	IF a[i+15:i] < min[15:0]
		index[2:0] := j
		min[15:0] := a[i+15:i]
	FI
ENDFOR
dst[15:0] := min[15:0]
dst[18:16] := index[2:0]
dst[127:19] := 0

[llvmbot]

Hi!

This issue may be a good introductory issue for people new to working on LLVM. If you would like to work on this issue, your first steps are:

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3. Fix the issue locally.
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7. Open a pull request to the upstream repository on GitHub. Detailed instructions can be found in GitHub's documentation. Mention this issue in the description of the pull request.

If you have any further questions about this issue, don't hesitate to ask via a comment in the thread below.

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: Simon Pilgrim (RKSimon)

_mm_minpos_epu16 ``` index[2:0] := 0 min[15:0] := a[15:0] FOR j := 0 to 7 i := j*16 IF a[i+15:i] < min[15:0] index[2:0] := j min[15:0] := a[i+15:i] FI ENDFOR dst[15:0] := min[15:0] dst[18:16] := index[2:0] dst[127:19] := 0 ```

[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Simon Pilgrim (RKSimon)

_mm_minpos_epu16 ``` index[2:0] := 0 min[15:0] := a[15:0] FOR j := 0 to 7 i := j*16 IF a[i+15:i] < min[15:0] index[2:0] := j min[15:0] := a[i+15:i] FI ENDFOR dst[15:0] := min[15:0] dst[18:16] := index[2:0] dst[127:19] := 0 ```

[kimsh02]

@RKSimon I'd like to take this.