[liftoff] Shorten names of value types

Import the most commonly used ValueType kinds as constants into
{LiftoffCompiler}, so that we don't need the fully-qualified accessor on
{ValueType}.
This makes many template instantiations much shorter and hence easier to
read.

Drive-by: Replace break after function by return. This often saves
  another line, making macros and switch statements much shorter.

R=zhin@chromium.org

Bug: v8:10364
Change-Id: If78352c2fa6b854989fa82718d0b3478b4d73aad
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2129634
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66943}