[DAGCombine] Don't combine sext with extload if sextload is not supported and...

[DAGCombine] Don't combine sext with extload if sextload is not supported and extload has multi users

In function DAGCombiner::visitSIGN_EXTEND_INREG, sext can be combined with extload even if sextload is not supported by target, then

  if sext is the only user of extload, there is no big difference, no harm no benefit.
  if extload has more than one user, the combined sextload may block extload from combining with other zext, causes extra zext instructions generated. As demonstrated by the attached test case.

This patch add the constraint that when sextload is not supported by target, sext can only be combined with extload if it is the only user of extload.

Differential Revision: https://reviews.llvm.org/D39108

llvm-svn: 316802