[flang] Add -f[no-]slp-vectorize flags (#132801)
Add -f[no-]slp-vectorize to the flang driver.
Add corresponding -fvectorize-slp to the flang frontend.
Enable -fslp-vectorize at -O2 and higher in flang to match the current
behaviour in clang.
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Signed-off-by: Kajetan Puchalski <kajetan.puchalski@arm.com>
