[WebAssembly] Fix liveness tracking info after drop insertion

Summary:
This fixes liveness tracking information after `drop` instruction
insertion in ExplicitLocals pass.

When a drop instruction is inserted to drop a dead register operand, the
original operand should be marked not dead anymore because it is now
used by the new drop instruction. And the operand to the new drop
instruction should be marked killed instead. This bug caused some
programs to fail when `llc` is run with `-verify-machineinstrs` option.

Reviewers: dschuff

Subscribers: sbc100, jgravelle-google, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D48253

llvm-svn: 335074