[InstCombine][NFC] masked-merge: add 'and' tests, too.

(and plain 'or', for completeness sake.)

After submitting D45631, i have realized that it will *already*
affect 'and' pattern, and it was obvious that there were no
good test patterns to show that.

Since the masked-merge.ll is getting kinda big,
unify naming schemes a bit, and split into 'xor'/'and'/'or'
testfiles, with the only difference being the last operation.

llvm-svn: 330072