Add mulx support to Math.BigMul

[Daniel-Svensson]

Follow up to #117261, with added support for mulx to hopefully adress performance regression #128594.

@EgorBo I've not yet run the benchmarks, is it easy for you to use your bot against a PR ?
Otherwise you will have to wait for a couple of days for benchmarks results

Corrently both lokal spmi run and
devops build shows an expected improvement to generated code.

Devops spmi results shows nice improvements especially for Linux x64 with >5000 bytes improvement with memory accesses in several locations.

local SPMI results (latest)

FullOpts (-1,488 bytes)

Collection	Base size (bytes)	Diff size (bytes)	PerfScore in Diffs
aspnet2.run.windows.x64.checked.mch	1,958,162	-37	-2.39%
benchmarks.run.windows.x64.checked.mch	17,296,800	-186	-4.61%
benchmarks.run_pgo.windows.x64.checked.mch	17,724,325	-997	-1.59%
benchmarks.run_pgo_optrepeat.windows.x64.checked.mch	16,067,339	-186	-4.61%
libraries.crossgen2.windows.x64.checked.mch	41,522,433	-121	-3.59%
libraries.pmi.windows.x64.checked.mch	64,543,375	+194	-2.43%
libraries_tests.run.windows.x64.Release.mch	126,045,916	-103	-1.64%
libraries_tests_no_tiered_compilation.run.windows.x64.Release.mch	151,916,143	+62	-1.87%
realworld.run.windows.x64.checked.mch	10,544,362	-54	-7.47%
smoke_tests.nativeaot.windows.x64.checked.mch	5,541,852	-60	-8.64%

local SPMI where op1 was fixed

FullOpts (-707 bytes)

Collection	Base size (bytes)	Diff size (bytes)	PerfScore in Diffs
aspnet2.run.windows.x64.checked.mch	1,958,162	-44	-2.55%
benchmarks.run.windows.x64.checked.mch	17,296,800	-161	-2.05%
benchmarks.run_pgo.windows.x64.checked.mch	17,724,325	-928	-0.44%
benchmarks.run_pgo_optrepeat.windows.x64.checked.mch	16,067,339	-161	-2.05%
libraries.crossgen2.windows.x64.checked.mch	41,522,433	-99	+1.35%
libraries.pmi.windows.x64.checked.mch	64,543,375	+676	+7.50%
libraries_tests.run.windows.x64.Release.mch	126,045,916	-306	+0.39%
libraries_tests_no_tiered_compilation.run.windows.x64.Release.mch	151,916,143	+420	-0.78%
realworld.run.windows.x64.checked.mch	10,544,362	-44	-6.64%
smoke_tests.nativeaot.windows.x64.checked.mch	5,541,852	-60	-8.64%

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