[PowerPC] Fix store-fptoi combine of f128 on Power8

llc would crash for (store (fptosi-f128-i32)) when -mcpu=pwr8, we should
not generate FP_TO_(S|U)INT_IN_VSR for f128 types at this time. This
patch fixes it.

Reviewed By: steven.zhang

Differential Revision: https://reviews.llvm.org/D86686