Fixup in test/DebugInfo/X86/live-debug-vars-discard-invalid.mir

The test case used invalid source operands as input
to BTS64rr instructions (feeding register operands with
immediates). This patch changes those instruction into
using BTS64ri8 instead, which seems to better match the
operand types.

Fixes problems seen in https://reviews.llvm.org/D63973.

llvm-svn: 369866