[NFC] [PowerPC] add an routine in PPCTargetLowering to determine if a global...

[NFC] [PowerPC] add an routine in PPCTargetLowering to determine if a global is accessed as got-indirect or not.

In theory, we should let the PPC target to determine how to lower the TOC Entry for globals. 
And the PPCTargetLowering requires this query to do some optimization for TOC_Entry. 

Differential Revision: https://reviews.llvm.org/D54925

llvm-svn: 348108