[SelectionDAG] Fix in legalization of UMAX/SMAX/UMIN/SMIN. Solves PR31486.

Summary:
Originally

 i64 = umax t8, Constant:i64<4>

was expanded into

 i32,i32 = umax Constant:i32<0>, Constant:i32<0>
 i32,i32 = umax t7, Constant:i32<4>

Now instead the two produced umax:es return i32 instead of i32, i32.

Thanks to Jan Vesely for help with the test case.

Patch by mikael.holmen at ericsson.com

Reviewers: bogner, jvesely, tstellarAMD, arsenm

Subscribers: test, wdng, RKSimon, arsenm, nhaehnle, llvm-commits

Differential Revision: https://reviews.llvm.org/D28135

llvm-svn: 291441