[TII] Split isTrivialReMaterializable into two versions [nfc] (#160377)

This change builds on https://github.com/llvm/llvm-project/pull/160319
which tries to clarify which *callers* (not backends) assume that the
result is actually trivial.

This change itself should be NFC. Essentially, I'm just renaming the
existing isTrivialRematerializable to the non-trivial version and then
adding a new trivial version (with the same name as the prior function)
and simplifying a few callers which want that semantic.

This change does *not* enable non-trivial remat any more broadly than
was already done for our targets which were lying through the old APIs;
that will come separately. The goal here is simply to make the code
easier to follow in terms of what assumptions are being made where.

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Co-authored-by: Luke Lau <luke_lau@icloud.com>