Consider constructor type when lifting decimal constants

[ElektroKill]

Link to issue(s) this covers:
Fixes #2949

Problem

ILSpy did not use the proper decimal constructor when transforming the LdcI4 instruction into a LdcDecimal instruction. Furthermore, the constructors with parameter type long and ulong were not supported at all as the pattern did not account for LdcI8.

Code to reproduce the missing constructor issue:

public static decimal LongToDecimal() {
	return (decimal)long.MaxValue;
}

public static decimal ULongToDecimal() {
	return (decimal)ulong.MaxValue;
}

Solution

• Ensure the correct decimal constructor is used when creating LdcDecimal instructions.
• Add support for the two aforementioned previously unsupported constructors.

[siegfriedpammer]

Would be nice if you could add the example code as test cases somewhere... Thanks!

[ElektroKill]

Would be nice if you could add the example code as test cases somewhere... Thanks!

I went with extending the correctness test for decimal since ILSpy will always decompile decimal constants using the raw value and will not introduce special constants like uint.MaxValue so a pretty test would not have been ideal.

[siegfriedpammer]

Thank you very much for the contribution!