[AMDGPU] Fixed WWM bug in block otherwise entirely in WQM

Summary:
For a block with WQM on entry and exit and containing no exact mode
code, but containing some WWM code, the WQM pass forgot to process the
block at all and so did not insert code to enter and leave WWM.

This commit fixes that.

Subscribers: arsenm, kzhuravl, wdng, nhaehnle, yaxunl, dstuttard, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D47027

Change-Id: I044792eead1293bed4203fb26ce75f47878afeb6
llvm-svn: 333362