X86: silence warning (-Wparentheses)

GCC 4.8.2 points out the ambiguity in evaluation of the assertion condition:

lib/Target/X86/X86FloatingPoint.cpp:949:49: warning: suggest parentheses around ‘&&’ within ‘||’ [-Wparentheses]
   assert(STReturns == 0 || isMask_32(STReturns) && N <= 2);

llvm-svn: 214672