Mips specific inline asm operand modifier 'M':

Print the high order register of a double word register operand.

In 32 bit mode, a 64 bit double word integer will be represented
by 2 32 bit registers. This modifier causes the high order register
to be used in the asm expression. It is useful if you are using 
doubles in assembler and continue to control register to variable
relationships.

This patch also fixes a related bug in a previous patch:

    case 'D': // Second part of a double word register operand
    case 'L': // Low order register of a double word register operand
    case 'M': // High order register of a double word register operand

I got 'D' and 'M' confused. The second part of a double word operand
will only match 'M' for one of the endianesses. I had 'L' and 'D'
be the opposite twins when 'L' and 'M' are.

llvm-svn: 160429