Seperate asmstring parsing from emission. This allows the code to be simpler

and more understandable.  It also allows us to do simple things like fold
consequtive literal strings together.  For example, instead of emitting this
for the X86 backend:

  O  << "adc" << "l" << " ";

we now generate this:

  O << "adcl ";

*whoa* :)

This shrinks the X86 asmwriters from 62729->58267 and 65176->58644 bytes
for the intel/att asm writers respectively.

llvm-svn: 19749