[PowerPC] Add combined ELF ABI and 32/64 bit queries to the subtarget. [NFC]

A lot of places in the code combine checks for both ABI (SVR4/Darwin/AIX) and
addressing mode (64-bit vs 32-bit). In an attempt to make some of the code more
readable I've added a couple functions that combine checking for the ELF abi and
64-bit/32-bit code at once. As we add more AIX support I intend to add similar
functions for the AIX ABI.

Differential Revision: https://reviews.llvm.org/D65814

llvm-svn: 369658
diff --git a/llvm/lib/Target/PowerPC/PPCFastISel.cpp b/llvm/lib/Target/PowerPC/PPCFastISel.cpp
index f524bf5..b22a459 100644
--- a/llvm/lib/Target/PowerPC/PPCFastISel.cpp
+++ b/llvm/lib/Target/PowerPC/PPCFastISel.cpp
@@ -2464,7 +2464,7 @@
                                 const TargetLibraryInfo *LibInfo) {
     // Only available on 64-bit ELF for now.
     const PPCSubtarget &Subtarget = FuncInfo.MF->getSubtarget<PPCSubtarget>();
-    if (Subtarget.isPPC64() && Subtarget.isSVR4ABI())
+    if (Subtarget.is64BitELFABI())
       return new PPCFastISel(FuncInfo, LibInfo);
     return nullptr;
   }