[RISCV] Use slideup to lower build_vector when all operand are (extract_element X, 0) (#154450)

The general lowering of build_vector starts with splatting the first
operand before sliding down other operands one-by-one. However, if the
every operands is an extract_element from the first vector element, we
could use the original _vector_ (source of extraction) from the last
build_vec operand as start value before sliding up other operands (in
reverse order) one-by-one. By doing so we can avoid the initial splat
and eliminate the vector to scalar movement later, which is something we
cannot do with vslidedown/vslide1down.

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Co-authored-by: Craig Topper <craig.topper@sifive.com>
Co-authored-by: Luke Lau <luke@igalia.com>

0 files changed, 0 insertions, 0 deletions