LegalizerHelper::lowerMemCpyFamily only execpts G_MEMCPY, G_MEMMOVE, and
G_MMSET.

Instead of reporting ___memmove as an implementation of memcpy,
make it unavailable and let the lowering logic consider memmove as
a fallback path.

This avoids a special case 1:N mapping for libcall implementations.

This avoids using report_fatal_error and standardizes the error
message in a subset of the error conditions.

If the pointer is aligned to more than the size of the vector, we can
widen the load up to next power of 2 size, as SDAG performs.

Some of the v3 tests are currently worse - those should be addressed in
other issues.

This helps keep the legalizer output easier to read, splitting each
instructions legalization into a separate block.

This is the bare minimum to get the intrinsic to compile for AMDGPU,
and it's not optimal. We need to follow along closer with the existing
G_FMINNUM/G_FMAXNUM with custom lowering to handle the IEEE=0 case
better.

Just re-use the existing lowering for the old semantics for
G_FMINNUM/G_FMAXNUM. This does not change G_FMINNUM/G_FMAXNUM's
treatment,
nor try to handle the general expansion without an underlying min/max
variant (or with G_FMINIMUM/G_FMAXIMUM).

GlobalISel was previously inefficient in handling bitreverses of vector
types. This deals with i16, i32, i64 vector types and converts them into
i8 bitreverses and rev instructions.

Handles bitreverse for vector types which were previously falling back
onto Selection DAG. Includes 8-bit element vectors greater than 128 bits
and less than 64 bits: <32 x i8>, <4 x i8>, and odd vector types: <9 x
i8>.