Implementation of easy intrinsics

[mariaKt]

This PR implements a few intrinsics that we identified missing from running the ui test suite.

Skip lists

• Sorted the skip list for consistent ordering
• Separated skip lists per backend (renamed skip.txt -> skip-prove.txt, added skip-exec.txt), since the two backends can have different pass/fail sets.
• Updated skip lists based on full runs of all 2,888 stable-mir-ui tests:
  • Haskell backend (skip-prove.txt): 1,159 pass, 1,729 fail
  • LLVM backend (skip-exec.txt): 1,164 pass, 1,724 fail
• Added test-stable-mir-ui-exec Makefile target for LLVM UI tests (renamed existing target to test-stable-mir-ui-prove)
• Enabled --update-skip for the LLVM test suite (test_stable_mir_ui_exec.py)
• Removed 2 tests that now pass on both backends after the intrinsic fixes below

New intrinsics

• assert_zero_valid — no-op, same as assert_inhabited (2 previously failing tests)
• rotate_left — bitwise left rotation using #rotateLeftInt helper (2 previously failing tests)
• bswap — byte swap using Int2Bytes/Bytes2Int endianness conversion (1 previously failing test)
• ctpop — population count using recursive #popcount helper (1 previously failing test)
• ctlz_nonzero — count leading zeros using #log2floor helper (1 previously failing test)

Of the 7 tests previously stuck on these intrinsics, 2 now pass end-to-end on both backends:

• rfcs/rfc-2294-if-let-guard/run-pass.rs (was stuck on rotate_left)
• consts/const-endianess.rs (was stuck on bswap)

The other 5 progress past the intrinsic but hit further issues.