Simplify operand padding by keying off tabs in the asm stream. If

Simplify operand padding by keying off tabs in the asm stream.  If
padding is disabled, tabs get replaced by spaces except in the case of
the first operand, where the tab is output to line up the operands after
the mnemonics.

Add some better comments and eliminate redundant code.

Fix some testcases to not assume tabs.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@77740 91177308-0d34-0410-b5e6-96231b3b80d8