Tune codegen for rb_yield() calls landing in ISeqs

Unlike in older revisions in the year, GCC 11 isn't inlining the call
to vm_push_frame() inside invoke_iseq_block_from_c() anymore. We do
want it to be inlined since rb_yield() speed is fairly important.
Logs from -fopt-info-optimized-inline reveal that GCC was blowing its
code size budget inlining invoke_block_from_c_bh() into its various
callers, leaving suboptimal code for its body.

Take away some uses of the `inline` keyword and merge a common tail
call to vm_exec() for overall better code.

This tweak gives about 18% on a micro benchmark and 1% on the
chunky-png benchmark from yjit-bench. I tested on a Skylake server.

```
$ cat c-to-ruby-call.yml
benchmark:
  - 0.upto(10_000_000) {}

$ benchmark-driver --chruby '+patch;master' c-to-ruby-call.yml
Warming up --------------------------------------
0.upto(10_000_000) {}      2.299 i/s -       3.000 times in 1.304689s (434.90ms/i)
Calculating -------------------------------------
                          +patch      master
0.upto(10_000_000) {}      2.299       1.943 i/s -       6.000 times in 2.609393s 3.088353s

Comparison:
             0.upto(10_000_000) {}
               +patch:         2.3 i/s
               master:         1.9 i/s - 1.18x  slower

$ ruby run_benchmarks.rb --chruby 'master;+patch' chunky-png
<snip>

----------  -----------  ----------  -----------  ----------  --------------  -------------
bench       master (ms)  stddev (%)  +patch (ms)  stddev (%)  +patch 1st itr  master/+patch
chunky-png  1156.1       0.1         1142.2       0.2         1.01            1.01
----------  -----------  ----------  -----------  ----------  --------------  -------------
```

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