Mark some branches as constexpr #2448

Job	Run time
arm (clang, aarch64, -Wno-psabi)	31m 38s
externals (clang, x86, -msse2)	45m 26s
arm (clang, arm, -Wno-psabi)	32m 52s
externals (gcc, x86, -msse2)	54m 16s
macosx (clang, x86_osx, -mavx)	1h 34m 42s
no_pch (clang, x86, -msse2)	58m 32s
nosimd (clang, x86, -DEVE_NO_SIMD)	38m 14s
no_pch (gcc, x86, -msse2)	1h 3m 51s
nosimd (gcc, x86, -DEVE_NO_SIMD)	48m 1s
riscv (clang, riscv, -mrvv-vector-bits=128)	33m 2s
x86-clang (clang, x86, -msse4.2)	18m 10s
x86-clang (clang, x86, -mavx)	23m 48s
x86-clang (clang, x86, -mavx2)	22m 57s
other-arch (gcc, sve256, -Wno-psabi)	1h 16m 21s
other-arch (gcc, sve512, -Wno-psabi)	2h 37m 59s
other-arch (gcc, ppc64, -Wno-psabi)	40m 9s
avx512 (clang, x86, -march=skylake-avx512)	37m 2s
avx512 (gcc, x86, -march=skylake-avx512)	55m 24s
other-cases (clang_libcpp, x86, -msse2)	23m 43s
other-cases (clang, x86, -mavx2 -mbmi2 -DEVE_USE_BMI_ON_AVX2)	28m 23s
other-cases (gcc, x86, -mavx2 -mbmi2 -DEVE_USE_BMI_ON_AVX2)	50m 47s
x86-gcc (gcc, x86, -msse4.2)	15m 28s
x86-gcc (gcc, x86, -mavx)	18m 2s
x86-gcc (gcc, x86, -mavx2)	21m 37s
	18h 10m 24s

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