Speed up setting instance variables

[tenderlove]

This patch speeds up setting instance variables somewhat. Here is the
benchmark:

class IVSetter
  def init
    @a = @b = @c = @d = nil
  end
end

obj = IVSetter.new

require "benchmark/ips"

Benchmark.ips do |x|
  x.report("init") do
    obj.init
  end
end

Before this patch:

[aaron@tc-lan-adapter ~/g/ruby (master)]$ ./ruby -v ../y.rb
ruby 2.8.0dev (2020-05-13T15:51:04Z master 65c5a39578) [x86_64-darwin19]
Warming up --------------------------------------
                init     1.324M i/100ms
Calculating -------------------------------------
                init     13.034M (± 1.6%) i/s -     66.210M in   5.080905s

After this patch:

[aaron@tc-lan-adapter ~/g/ruby (faster-ivar-set)]$ ./ruby -v ../y.rb
ruby 2.8.0dev (2020-05-14T19:33:13Z faster-ivar-set 60cfbb9d7e) [x86_64-darwin19]
Warming up --------------------------------------
                init     1.456M i/100ms
Calculating -------------------------------------
                init     14.599M (± 1.5%) i/s -     74.270M in   5.088467s

This speeds up ivar sets because we can reduce redundant checks.
ROBJECT_NUMIV and ROBJECT_IVPTR both check that the object is a
T_OBJECT, but the caller (vm_setivar) already knows this is a
T_OBJECT (otherwise this branch wouldn't be executing).

Secondly, both ROBJECT_IVPTR and ROBJECT_NUMIV check flags for
ROBJECT_EMBED. We can reduce the check to just once.

I thought the C compiler would be smart enough to eliminate these
redundant checks, but after reading the assembly code it's clear that
the checks are being performed multiple times.

[tenderlove]

If we enable NDEBUG, performance is completely different and this patch has no impact, so I'm closing.