Avoid repeated casts in builtins.lp64/llp64 tests to reduce SemIR size (#6702)
This reduces the size of a couple large test files by a few hundred
lines:
```
toolchain/check/testdata/interop/cpp/builtins.llp64.carbon | 4033 +++++++++++++++++++++---------------------------
toolchain/check/testdata/interop/cpp/builtins.lp64.carbon | 4019 ++++++++++++++++++++---------------------------
2 files changed, 3355 insertions(+), 4697 deletions(-)
```
Nicholas Bishop