[X86] Move integer hadd/hsub formation into a helper function shared by combineAdd and combineSub.

There was a lot of duplicate code here for checking the VT and
subtarget. Moving it into a helper avoids that.

It also fixes a bug that combineAdd reused Op0/Op1 after a call
to isHorizontalBinOp may have changed it. The new helper function
has its own local version of Op0/Op1 that aren't shared by other
code.

Fixes PR46455.

Reviewed By: spatel, bkramer

Differential Revision: https://reviews.llvm.org/D83971

(cherry picked from commit 5408024f)