[AsmPrinter] Remove const qualifier from EmitBasicBlockStart.

Overriders may want to modify state in it. AMDGPU wants
to, but has to make its members mutable in order to do so.

Besides, EmitBasicBlockEnd is not const, so why should
Start be?

Patch by Bevin Hansson.

Reviewed By: nickdesaulniers

Differential Revision: https://reviews.llvm.org/D66341

llvm-svn: 369325