The changes are:
1) Pass lambdas as const references to avoid unnecessary copying.
2) Remove CodeStubAssembler* parameter from loop bodies and let the lambdas
   capture the right assembler instead. It makes the loop body code look
   uniform with the surrounding code and unblocks splitting of a CSA
   into different classes.

BUG=

Review-Url: https://codereview.chromium.org/2535753012
Cr-Commit-Position: refs/heads/master@{#41482}