[PowerPC] Fix assert from machine verify pass that atomic pseudo expanding...

[PowerPC] Fix assert from machine verify pass that atomic pseudo expanding causes mismatched register class

For atomic value operand which less than 4 bytes need to be masked. 
And the related operation to calculate the newvalue can be done in 32 bit gprc. 
So just use gprc for mask and value calculation.

Differential Revision: https://reviews.llvm.org/D56077

llvm-svn: 350113