[PowerPC] Fix the assert of combineBVOfConsecutiveLoads when element num is 1

Building a vector out of multiple loads can be converted to a load of the vector type if the loads are consecutive.
But the special condition is that the element number is 1, such as <1 x i128>. So just early exit to fix the assert.

Patch By: wuzish (Zixuan Wu)
Differential Revision: https://reviews.llvm.org/D52072

llvm-svn: 342611