fix[lang]: fix array index checks when the subscript is folded

[charles-cooper]

What I did

fix a regression introduced in #3669. repro:

@external
def foo(array: int128[10]) -> int128:
    return array[0 - 1]  # should get caught at compile-time, but fails at runtime instead

credit @trocher for reporting

How I did it

How to verify it

Commit message

this commit fixes a regression introduced in 56c4c9dbc. prior to
56c4c9dbc, the folded subscript would be checked for OOB access, but
after 56c4c9dbc, expressions like `foo[0 - 1]` can slip past the
typechecker (getting demoted to a runtime check). also, a common pattern
is refactored.

common pattern:
```python
if node.has_folded_value:
    node = node.get_folded_value()
```
=>
```
node = node.reduced()
```

Description for the changelog

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 51.52%. Comparing base (b43ffac) to head (8f46cd1).

Files	Patch %	Lines
vyper/semantics/types/subscriptable.py	0.00%	4 Missing ⚠️
vyper/semantics/analysis/local.py	33.33%	2 Missing ⚠️
vyper/codegen/expr.py	50.00%	1 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           master    #3924       +/-   ##
===========================================
- Coverage   90.96%   51.52%   -39.45%     
===========================================
  Files          95       95               
  Lines       14397    14396        -1     
  Branches     3191     3187        -4     
===========================================
- Hits        13096     7417     -5679     
- Misses        901     6394     +5493     
- Partials      400      585      +185     

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[trocher]

lgtm, however, a similar regression is there for Tuples:

The following compiles in v0.3.10 but not on master

x:constant(uint256) = 1
@external
def foo():
    a:uint256 = 1
    b:uint256 = 1

    c:uint256 = (a,b)[x]
    v:uint256 = (a,b)[1 + 0]
    

see

vyper/vyper/semantics/types/subscriptable.py

Lines 369 to 375 in b43ffac

	def validate_index_type(self, node):
	if not isinstance(node, vy_ast.Int):
	raise InvalidType("Tuple indexes must be literals", node)
	if node.value < 0:
	raise ArrayIndexException("Vyper does not support negative indexing", node)
	if node.value >= self.length:
	raise ArrayIndexException("Index out of range", node)

The fix might be more involved as node.value is used in several places:

vyper/vyper/semantics/types/subscriptable.py

Lines 377 to 378 in b43ffac

	def get_subscripted_type(self, node):
	return self.member_types[node.value]