Fix skipping deleted instructions before replacing operands.
Fix skipping deleted instructions before replacing operands.

Fixes hitting a (benign) assert in replaceSource().

Change-Id: I7f984d484133e619717d004f20cd671a54473185
Reviewed-on: https://chromium-review.googlesource.com/414490
Reviewed-by: Jim Stichnoth <46a13f57ad465cec7f48cfe1a20c394bb659ab25@chromium.org>

google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader