x86: Add missing break to getCallPreservedMask()

This change brings getCallPreservedMask()'s logic in line with
getCalleeSavedRegs().

While this changes the control flow slightly, the change is not
currently observable.  is64Bit must be false to get to the accidental
fallthrough, but the case that we fall into (coldcc) does nothing unless
is64Bit is true.

llvm-svn: 203943