Optimize Java Lite code generation for message field getters

[copybara-service]

Optimize Java Lite code generation for message field getters

Read the message field into a local variable before checking for null
and returning the default instance or the field value.

In D8 debug mode (dev builds), ternary expressions on fields emit two
separate iget-object instructions and an extra goto branch. Reading to a
local variable ensures only a single iget-object instruction is emitted
and eliminates the extra branch in dev dex bytecode.

Before:

public Header getHeader() {
  return header_ == null ? Header.getDefaultInstance() : header_;
}

.method public getHeader()LHeader;
    iget-object v0, p0, LMyProto;->header_:LHeader;     # Read 1
    if-nez v0, :cond_9
    invoke-virtual {p0}, LMyProto;->getDefaultInstance()LHeader;
    move-result-object v0
    goto :goto_b                                       # Extra branch jump
    :cond_9
    iget-object v0, p0, LMyProto;->header_:LHeader;     # Read 2 (redundant)
    :goto_b
    return-object v0
.end method

After:

.method public getHeader()LHeader;
    iget-object v0, p0, LMyProto;->header_:LHeader;     # Exactly 1 iget-object
    if-nez v0, :cond_8
    invoke-virtual {p0}, LMyProto;->getDefaultInstance()LHeader;
    move-result-object v0
    :cond_8
    return-object v0                                    # 0 goto jumps
.end method