Eliminate Intermediate::replace()

Eliminate Intermediate::replace()

With Reactor variables now merely tracking the last assigned rvalue if
used within a single basic block, we can avoid the unsafe replace()
operation on SpirvShader::Intermediate.

This effectively reverts
https://swiftshader-review.googlesource.com/c/SwiftShader/+/27769

Bug b/129356087
Bug b/128527271

Change-Id: Ibfafc6960ac7e10b898ff8804752b928a7fc1988
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/28109
Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>