[PowerPC] Replace cntlz[.] with cntlzw[.]

cntlz is the old POWER mnemonic. cntlzw is the PowerPC mnemonic.

This change fixes an issue when -no-integrated-as: The opcode cntlz is
unrecognized by gas

Alias the POWER mnemonic cntlz[.] to the PowerPC mnemonic cntlzw[.]
This is done for because the POWER cntlz mnemonic has be used by LLVM for
a very long time. We need to make sure that assembly programs
that are using the cntlz[.] do not break with this change.

Change PowerPC tests to reflect the insn change from cntlz to cntlzw.
Add assembly test to verify cntlz[.] is encoded correctly.

Patch by Tom Rix!

llvm-svn: 251489