[tint][hlsl] Extract ternary value to let if necessary

In the HLSL backend, WGSL selects are lowered to ternary operators
(using HLSL 2018), which should match the non-short-circuiting semantics
of select. However, in the case that the condition is a constant bool
value, a buggy optimization in DXC may constant fold away the ternary
operator, resulting in one side of the select not being evaluated. This
causes incorrect behavior if the un-selected value would have had side
effects. To prevent this bad optimization from being hit, we extract the
potentially affected argument to a let variables to force its evaluation
to occur.

Bug: 478792488
Change-Id: I227cb92e175e5659ddb30b662dd8273cc0e92c9d
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/287359
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Reviewed-by: James Price <jrprice@google.com>
Commit-Queue: Natalie Chouinard <chouinard@google.com>