Reduce ExecSeqScan* code size using pg_assume()

Reduce ExecSeqScan* code size using pg_assume()

fb9f955025f optimized code generation by using specialized variants of
ExecSeqScan* for [not] having a qual, projection etc. This allowed the
compiler to optimize the code out the code for qual / projection. However, as
observed by David Rowley at the time, the compiler couldn't prove the
opposite, i.e. that the qual etc *are* present.

By using pg_assume(), introduced in d65eb5b1b84, we can tell the compiler that
the relevant variables are non-null.

This reduces the code size to a surprising degree and seems to lead to a small
but reproducible performance gain.

Reviewed-by: Amit Langote <amitlangote09@gmail.com> Discussion:
https://postgr.es/m/CA+HiwqFk-MbwhfX_kucxzL8zLmjEt9MMcHi2YF=DyhPrSjsBEA@mail.gmail.com