Torque expressions of type constexpr are evaluated at compile-time
rather than runtime. They are backed by C++ types rather than
TNode<X> types, so the macro functions that are called by generated
C++ code expect values to be computed when the snapshot is generated
rather than by TurboFan-generated code.

Specifically, "if" statements can have a constexpr modifier. With this
modifier, a type of "constexpr bool" is expected rather than "bool",
and in that case instead of generating a CSA BranchIf, it generates
a C++ "if (<bool expression>)" that generates code for only the true or
false path based on the bool value at torque-execution (compile time)
rather than generating both paths (including inserting phi nodes
for variables modified on either branch at the re-merge at the end
of the if) and dynamically dispatching to the true or false path
during d8/Chrome/node.js execution (runtime) using a CSA BranchIf.

Change-Id: I8238e25aaadbfc618847e04556e96a3949ea5a8d
Reviewed-on: https://chromium-review.googlesource.com/1042085
Commit-Queue: Daniel Clifford <danno@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53001}