nir: Lower int64 in a single pass.

This commit also refactors min/max lowering into one function.
10 jobs for !1224 with lower_int64 in 13 minutes and 22 seconds (queued for 12 seconds)
latest detached
Status Job ID Name Coverage
  Containers Build
passed debian #414799

00:00:24

 
  Build+Test
passed meson-clang #414801

00:08:58

passed meson-clover #414805

00:07:02

passed meson-main #414804

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passed meson-swr-glvnd #414800

00:06:18

passed meson-vulkan #414806

00:01:50

passed scons-llvm #414808

00:04:24

passed scons-nollvm #414807

00:03:59

passed scons-swr #414802

00:07:50

passed scons-win64 #414803

00:09:57