[SelectionDAG] Remove the code that attempts to calculate the alignment for...

[SelectionDAG] Remove the code that attempts to calculate the alignment for the second half of a split masked load/store.

The code divides the alignment by 2 if the original alignment is
equal to the original VT size. But this wouldn't be correct
if the alignment was larger than the VT size.

The memory operand object already takes care of calling MinAlign
on the base alignment and the memory pointer offset. So we don't
need any special code at all.

llvm-svn: 364151