Updates the prologue_offset value in LCodeGen::GeneratePrologue after
generating ahi/aghi.
prologue_offset is used in FindCodeAgeSequence. Failing to update
prologue_offset will result in failing to correctly identify the code
aging sequence (young).

R=joransiu@ca.ibm.com, mbrandy@us.ibm.com, michael_dawson@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1843673002

Cr-Commit-Position: refs/heads/master@{#35112}