[PowerPC] Add parentheses to silence gcc warning

Without gcc 7.4 warns with

../lib/Target/PowerPC/PPCInstrInfo.cpp:2284:25: warning: suggest parentheses around '&&' within '||' [-Wparentheses]
          BaseOp1.isFI() &&
          ~~~~~~~~~~~~~~~^~
              "Only base registers and frame indices are supported.");
              ~