[AArch64] Fix || Add brackets for || inside of assert
Fixes:
llvm/lib/Target/AArch64/AArch64GenRegisterBankInfo.def:185:12: warning: suggest parentheses around ‘&&’ within ‘||’ [-Wparentheses]
assert(!Size.isScalable() || MinSize >= 128
~~~~~~~~~~~~~~
&& "Scalable vector types should have size of at least 128 bits");
