Revert r289638: [PowerPC] Fix logic dealing with nop after calls (and tail-call eligibility)

This patch appears to result in trampolines in vtables being miscompiled
when they in turn tail call a method.

I've posted some preliminary details about the failure on the thread for
this commit and talked to Hal. He was comfortable going ahead and
reverting until we sort out what is wrong.

llvm-svn: 289928