[AMDGPU] Fix s_branch with -1 offset

Summary:
In case s_branch instruction target is itself backend should emit offset -1 but instead it emit 0.
'''
label:
    s_branch label  // should emit [0xff,0xff,0x82,0xbf]
'''

Tom, Matt: why are we adjusting fixup values in applyFixup() method instead of processFixup()? processFixup() is calling adjustFixupValue() but does nothing with its result.

Reviewers: vpykhtin, artem.tamazov, tstellarAMD

Subscribers: arsenm, kzhuravl, wdng, nhaehnle, yaxunl

Differential Revision: https://reviews.llvm.org/D24671

llvm-svn: 281896