[MSP430] Fix absolute addressing mode printing in AsmPrinter

Align checks for absolute addressing mode with its current
implementation (SR is used as a base register).

This fixes https://bugs.llvm.org/show_bug.cgi?id=39993

Patch by Kristina Bessonova!

Differential Revision: https://reviews.llvm.org/D56785

llvm-svn: 352178