startswith/endswith wrap starts_with/ends_with and will eventually go away (to more closely match string_view)

6f9e743b91ad6ac1f333c introduced a mode which renames only instructions in
the function. This change updates that mode to skip instructions that are already named.
This serves the original purpose of the mode (rename-only-inst) which is:
1. Modify IR without failing verifier with serially ordered number
   requirement (%1, %2, %3 required in order).
2. Give meaningful names to instructions.

This is useful when needing to modify IR and test some optimizations on
them, while keeping BB names and function names intact. If
the IR uses ordered number naming (%1, %2, %3 etc), then we cannot just
remove or reorder specific instructions since the verifier expects the
numbers to be in order.

A new attempt after removing uses of -instnamer in polly lit tests
in D148530.

This reverts commit 981ec1faeb508a364cc47c8246b72fc89dd8c1d8.

It broke polly build bots. Polly still uses -instnamer with legacy PM.

This should improve readability of metarenamed code.

Differential Revision: https://reviews.llvm.org/D145896
Reviewed By: nikic

Currently we use 'tmp', which is also a keyword for FileCheck. It leads to this
annoying warning whenever a script for auto-generation of checks is used.
It is especially annoying that it happens to every test affected by metarenamer.

Just use another prefix for metarenamed names to avoid this.

Differential Revision: https://reviews.llvm.org/D144001
Reviewed By: nikic

LLVM contains a helpful function for getting the size of a C-style
array: `llvm::array_lengthof`. This is useful prior to C++17, but not as
helpful for C++17 or later: `std::size` already has support for C-style
arrays.

Change call sites to use `std::size` instead.

Differential Revision: https://reviews.llvm.org/D133429

This reverts commit ef8206320769ad31422a803a0d6de6077fd231d2.

- It conflicts with the existing llvm::size in STLExtras, which will now
  never be called.
- Calling it without llvm:: breaks C++17 compat