This is so that we don't expand to include unneeded 0 checks.

Also fix the logic error in LegalizerInfo so it is NOT legal on Thumb1
in Fast-ISEL.

Finally, Remove the README entry regarding this issue.

Implement lowering intrinsic `reset_fpmode` in Global Selector for ARM
target.

ARM uses complex encoding of immediate values using small number of
bits. As a result, some values cannot be represented as immediate
operands, they need to be synthesized in a register. This change
implements legalization of such constants with loading values from
constant pool.

---------

Co-authored-by: Matt Arsenault <arsenm2@gmail.com>

Implement handling of get/set floating point control modes for ARM in
Global Instruction Selector.

I've been looking at LegacyLegalizerInfo and what its place in GISel is.
It seems like it's very close to being deleted so I'm checking if we can
remove the last remaining uses of it.

Looks like we can do a drop-in replacement with the new legalizer for
ARM.

Implement handling of get/set floating point environment for ARM in
Global Instruction Selector. Lowering of these intrinsics to operations
on FPSCR was previously inplemented in DAG selector, in GlobalISel it is
reused.

This tries to allow libcalls to be tail called, using a similar method
to DAG where the type is checked to make sure they match, and if so the
backend, through lowerCall checks that the tailcall is valid for all
arguments.

Before restoring `CodeGen/LowLevelType`, rename this to `LowLevelTypeUtils`.

Differential Revision: https://reviews.llvm.org/D148768

Discourse thread: https://discourse.llvm.org/t/include-what-you-use-include-cleanup
Differential Revision: https://reviews.llvm.org/D121332